Reland of Convert enable_pre_sync_backup flag to new flags system. (patchset #1 id:1 of https://codereview.chromium.org/1490713002/ )

Reason for revert:
Speculative revert did not help. Relanding.

Original issue's description:
> Revert of Convert enable_pre_sync_backup flag to new flags system. (patchset #2 id:20001 of https://codereview.chromium.org/1487873003/ )
>
> Reason for revert:
> Speculative revert, CrOS compile broken, more info on the bug.
> BUG=563927
>
> Original issue's description:
> > Convert enable_pre_sync_backup flag to new flags system.
> >
> > This flag's usage is very isolated so is a good candidate for the new system. There is now a generated header for the #define rather than a global define.
> >
> > Reland of http://crrev.com/1478623004#ps60001
> > TBR=zea@chromium.org
> >
> > Committed: https://crrev.com/db7f9756dc6e236589b90c76d6fa0f1244601e78
> > Cr-Commit-Position: refs/heads/master@{#362365}
>
> TBR=zea@chromium.org,brettw@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/3c201fa28b53246177fed8d6b9454da4b0625df2
> Cr-Commit-Position: refs/heads/master@{#362368}

TBR=zea@chromium.org,brettw@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=563927

Review URL: https://codereview.chromium.org/1490723002

Cr-Commit-Position: refs/heads/master@{#362369}

7 files changed, 46 insertions, 7 deletions

diff --git a/components/browser_sync.gypi b/components/browser_sync.gypi
index 03393c7..6abdffd 100644
--- a/components/browser_sync.gypi
+++ b/components/browser_sync.gypi
@@ -30,6 +30,7 @@
         'syncable_prefs',
         'sync_bookmarks',
         'sync_driver',
+        'sync_driver_features',
         'sync_sessions',
         'variations',
         'version_info',
diff --git a/components/browser_sync/browser/profile_sync_service.cc b/components/browser_sync/browser/profile_sync_service.cc
index 4ace523..04cb904 100644
--- a/components/browser_sync/browser/profile_sync_service.cc
+++ b/components/browser_sync/browser/profile_sync_service.cc
@@ -49,6 +49,7 @@
 #include "components/sync_driver/signin_manager_wrapper.h"
 #include "components/sync_driver/sync_api_component_factory.h"
 #include "components/sync_driver/sync_client.h"
+#include "components/sync_driver/sync_driver_features.h"
 #include "components/sync_driver/sync_driver_switches.h"
 #include "components/sync_driver/sync_error_controller.h"
 #include "components/sync_driver/sync_stopped_reporter.h"
@@ -338,7 +339,7 @@ void ProfileSyncService::Initialize() {
     need_backup_ = false;
   }
 
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
   if (!running_rollback && !IsSignedIn()) {
     CleanUpBackup();
   }
@@ -2566,7 +2567,7 @@ void ProfileSyncService::ClearBrowsingDataSinceFirstSync() {
 void ProfileSyncService::CheckSyncBackupIfNeeded() {
   DCHECK_EQ(backend_mode_, SYNC);
 
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
   const base::Time last_synced_time = sync_prefs_.GetLastSyncedTime();
   // Check backup once a day.
   if (!last_backup_time_ &&
diff --git a/components/components_tests.gyp b/components/components_tests.gyp
index 2a08ccee..bf7b8d9 100644
--- a/components/components_tests.gyp
+++ b/components/components_tests.gyp
@@ -1050,6 +1050,7 @@
         'components.gyp:ssl_errors',
         'components.gyp:suggestions',
         'components.gyp:sync_bookmarks',
+        'components.gyp:sync_driver_features',
         'components.gyp:sync_driver_test_support',
         'components.gyp:sync_sessions',
         'components.gyp:sync_sessions_test_support',
diff --git a/components/sync_driver.gypi b/components/sync_driver.gypi
index 7bda955..fa4aeb3 100644
--- a/components/sync_driver.gypi
+++ b/components/sync_driver.gypi
@@ -18,6 +18,7 @@
         'invalidation_public',
         'os_crypt',
         'signin_core_browser',
+        'sync_driver_features',
         'version_info',
       ],
       'include_dirs': [
@@ -152,6 +153,26 @@
       ],
     },
     {
+      # GN version: //components/sync_driver:features
+      'target_name': 'sync_driver_features',
+      'includes': [ '../build/buildflag_header.gypi' ],
+      'variables': {
+        'buildflag_header_path': 'components/sync_driver/sync_driver_features.h',
+        'variables': {
+          'conditions': [
+            ['OS=="win" or OS=="mac" or (OS=="linux" and chromeos==0)', {
+              'enable_pre_sync_backup%': 1,
+            }, {
+              'enable_pre_sync_backup%': 0,
+            }],
+          ],
+        },
+        'buildflag_flags': [
+          "ENABLE_PRE_SYNC_BACKUP=<(enable_pre_sync_backup)",
+        ],
+      },
+    },
+    {
       'target_name': 'sync_driver_test_support',
       'type': 'static_library',
       'dependencies': [
diff --git a/components/sync_driver/BUILD.gn b/components/sync_driver/BUILD.gn
index ffe41b8..38b89a8 100644
--- a/components/sync_driver/BUILD.gn
+++ b/components/sync_driver/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
 import("//build/config/features.gni")
 
 source_set("sync_driver") {
@@ -120,6 +121,9 @@ source_set("sync_driver") {
     "user_selectable_sync_type.h",
   ]
 
+  public_deps = [
+    ":features",
+  ]
   deps = [
     "//base",
     "//base:prefs",
@@ -250,3 +254,12 @@ source_set("unit_tests") {
     ]
   }
 }
+
+buildflag_header("features") {
+  header = "sync_driver_features.h"
+
+  # Whether to back up data before sync.
+  enable_pre_sync_backup = is_win || is_mac || (is_linux && !is_chromeos)
+
+  flags = [ "ENABLE_PRE_SYNC_BACKUP=$enable_pre_sync_backup" ]
+}
diff --git a/components/sync_driver/backup_rollback_controller.cc b/components/sync_driver/backup_rollback_controller.cc
index b33bd60..3629973 100644
--- a/components/sync_driver/backup_rollback_controller.cc
+++ b/components/sync_driver/backup_rollback_controller.cc
@@ -12,12 +12,13 @@
 #include "base/single_thread_task_runner.h"
 #include "base/thread_task_runner_handle.h"
 #include "components/sync_driver/signin_manager_wrapper.h"
+#include "components/sync_driver/sync_driver_features.h"
 #include "components/sync_driver/sync_driver_switches.h"
 #include "components/sync_driver/sync_prefs.h"
 
 namespace sync_driver {
 
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
 // Number of rollback attempts to try before giving up.
 static const int kRollbackLimits = 3;
 
@@ -71,20 +72,20 @@ bool BackupRollbackController::StartRollback() {
 }
 
 void BackupRollbackController::OnRollbackReceived() {
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
   sync_prefs_->SetRemainingRollbackTries(kRollbackLimits);
 #endif
 }
 
 void BackupRollbackController::OnRollbackDone() {
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
   sync_prefs_->SetRemainingRollbackTries(0);
 #endif
 }
 
 // static
 bool BackupRollbackController::IsBackupEnabled() {
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
   const std::string group_name =
       base::FieldTrialList::FindFullName(kSyncBackupFinchName);
 
diff --git a/components/sync_driver/backup_rollback_controller_unittest.cc b/components/sync_driver/backup_rollback_controller_unittest.cc
index 9b94f21..7d5f9b6 100644
--- a/components/sync_driver/backup_rollback_controller_unittest.cc
+++ b/components/sync_driver/backup_rollback_controller_unittest.cc
@@ -12,6 +12,7 @@
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "components/sync_driver/signin_manager_wrapper.h"
+#include "components/sync_driver/sync_driver_features.h"
 #include "components/sync_driver/sync_driver_switches.h"
 #include "components/sync_driver/sync_prefs.h"
 #include "testing/gmock/include/gmock/gmock.h"
@@ -21,7 +22,7 @@ using ::testing::Return;
 
 namespace {
 
-#if defined(ENABLE_PRE_SYNC_BACKUP)
+#if BUILDFLAG(ENABLE_PRE_SYNC_BACKUP)
 
 class MockSigninManagerWrapper : public SigninManagerWrapper {
  public: