Add support for MojoChannel field trial

This adds support for the upcoming MojoChannel field trial.

It also simplifies the logic around enabling Mojo channel
in general, getting rid of compile-time overrides and instead
controlling the feature only at runtime via a single flag or
(in the absence of said flag) the field trial.

BUG=595082
R=jam@chromium.org

Review URL: https://codereview.chromium.org/1805653003

Cr-Commit-Position: refs/heads/master@{#381490}

5 files changed, 19 insertions, 28 deletions

diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index 16eb322..7d17955 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -1428,7 +1428,6 @@ void RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer(
     switches::kDisableLocalStorage,
     switches::kDisableLogging,
     switches::kDisableMediaSuspend,
-    switches::kDisableMojoChannel,
     switches::kDisableNotifications,
     switches::kDisableOverlayScrollbar,
     switches::kDisablePermissionsAPI,
@@ -1462,6 +1461,7 @@ void RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer(
     switches::kEnableLinkDisambiguationPopup,
     switches::kEnableLowResTiling,
     switches::kEnableMediaSuspend,
+    switches::kEnableMojoChannel,
     switches::kEnableInbandTextTracks,
     switches::kEnableLCDText,
     switches::kEnableLogging,
@@ -1474,7 +1474,6 @@ void RenderProcessHostImpl::PropagateBrowserCommandLineToRenderer(
     switches::kEnablePreciseMemoryInfo,
     switches::kEnablePreferCompositingToLCDText,
     switches::kEnableRGBA4444Textures,
-    switches::kEnableRendererMojoChannel,
     switches::kEnableSkiaBenchmarking,
     switches::kEnableSlimmingPaintV2,
     switches::kEnableSmoothScrolling,
diff --git a/content/public/common/mojo_channel_switches.cc b/content/public/common/mojo_channel_switches.cc
index a94e195..f649eb8 100644
--- a/content/public/common/mojo_channel_switches.cc
+++ b/content/public/common/mojo_channel_switches.cc
@@ -5,36 +5,39 @@
 #include "content/public/common/mojo_channel_switches.h"
 
 #include "base/command_line.h"
+#include "base/metrics/field_trial.h"
 #include "ipc/mojo/ipc_channel_mojo.h"
 #include "mojo/common/common_type_converters.h"
 
 namespace switches {
 
-// Replaces renderer-browser IPC channel with ChnanelMojo.
-// TODO(morrita): Now ChannelMojo for the renderer is on by default.
-// Remove this once the change sticks.
-const char kEnableRendererMojoChannel[] =
-    "enable-renderer-mojo-channel";
-
-// Disable ChannelMojo usage regardless of the platform or the process type.
-const char kDisableMojoChannel[] = "disable-mojo-channel";
+// Enable ChannelMojo on any supported platform.
+const char kEnableMojoChannel[] = "enable-mojo-channel";
 
 // The token to use to construct the message pipe on which to layer ChannelMojo.
 const char kMojoChannelToken[] = "mojo-channel-token";
 
 }  // namespace switches
 
+namespace {
+
+const char kMojoChannelExperimentName[] = "MojoChannel";
+
+}  // namespace
+
 namespace content {
 
 bool ShouldUseMojoChannel() {
-  const base::CommandLine& command_line =
-      *base::CommandLine::ForCurrentProcess();
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kEnableMojoChannel))
+    return true;
 
-  if (command_line.HasSwitch(switches::kDisableMojoChannel))
-    return false;
-  if (command_line.HasSwitch(switches::kEnableRendererMojoChannel))
+  const std::string group =
+      base::FieldTrialList::FindFullName(kMojoChannelExperimentName);
+  if (group == "Enabled")
     return true;
-  return IPC::ChannelMojo::ShouldBeUsed();
+
+  return false;
 }
 
 }  // namespace content
diff --git a/content/public/common/mojo_channel_switches.h b/content/public/common/mojo_channel_switches.h
index 1811843..20daf11 100644
--- a/content/public/common/mojo_channel_switches.h
+++ b/content/public/common/mojo_channel_switches.h
@@ -9,8 +9,7 @@
 
 namespace switches {
 
-extern const char kEnableRendererMojoChannel[];
-extern const char kDisableMojoChannel[];
+extern const char kEnableMojoChannel[];
 CONTENT_EXPORT extern const char kMojoChannelToken[];
 
 }  // namespace switches
diff --git a/ipc/mojo/ipc_channel_mojo.cc b/ipc/mojo/ipc_channel_mojo.cc
index 8832d8d..a0dabad 100644
--- a/ipc/mojo/ipc_channel_mojo.cc
+++ b/ipc/mojo/ipc_channel_mojo.cc
@@ -199,13 +199,6 @@ MojoResult UnwrapAttachment(mojom::SerializedHandlePtr handle,
 //------------------------------------------------------------------------------
 
 // static
-bool ChannelMojo::ShouldBeUsed() {
-  // TODO(rockot): Investigate performance bottlenecks and hopefully reenable
-  // this at some point. http://crbug.com/500019
-  return false;
-}
-
-// static
 scoped_ptr<ChannelMojo> ChannelMojo::Create(
     mojo::ScopedMessagePipeHandle handle,
     Mode mode,
diff --git a/ipc/mojo/ipc_channel_mojo.h b/ipc/mojo/ipc_channel_mojo.h
index c1b659a..8c5de24 100644
--- a/ipc/mojo/ipc_channel_mojo.h
+++ b/ipc/mojo/ipc_channel_mojo.h
@@ -38,9 +38,6 @@ class IPC_MOJO_EXPORT ChannelMojo
       public MojoBootstrap::Delegate,
       public NON_EXPORTED_BASE(internal::MessagePipeReader::Delegate) {
  public:
-  // True if ChannelMojo should be used regardless of the flag.
-  static bool ShouldBeUsed();
-
   // Creates a ChannelMojo.
   static scoped_ptr<ChannelMojo> Create(mojo::ScopedMessagePipeHandle handle,
                                         Mode mode,