Prepare for -Wall for third-party code, -Wextra for chromium_code

This lands patch set 16 of https://codereview.chromium.org/1551753002/
(reviewed there), except for the changes to build/common.gypi and
build/config/compiler/BUILD.gn.  The changes to these two files are likely
to break some bot I don't know about, and I don't want to keep reverting
and relanding all these files.  So this lands the parts that are hopefully
harmless separately.

BUG=573250
TBR=thestig

Review URL: https://codereview.chromium.org/1557733002

Cr-Commit-Position: refs/heads/master@{#367252}

1 files changed, 19 insertions, 1 deletions

diff --git a/third_party/mach_override/BUILD.gn b/third_party/mach_override/BUILD.gn
index 4672c46..ed1c331 100644
--- a/third_party/mach_override/BUILD.gn
+++ b/third_party/mach_override/BUILD.gn
@@ -2,6 +2,19 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+config("libudis86_warnings") {
+  if (is_clang) {
+    cflags = [
+      # syn.c contains a switch with an assert(false) in a default:
+      # block.  In release builds, the function is missing a return.
+      "-Wno-return-type",
+
+      # Fires once in decode.c.
+      "-Wno-sometimes-uninitialized",
+    ]
+  }
+}
+
 static_library("libudis86") {
   sources = [
     "libudis86/decode.c",
@@ -29,7 +42,12 @@ static_library("libudis86") {
   cflags = [ "-Wno-return-type" ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
-  configs += [ "//build/config/compiler:no_chromium_code" ]
+  configs += [
+    "//build/config/compiler:no_chromium_code",
+
+    # Must be after no_chromium_code for warning flags to be ordered correctly.
+    ":libudis86_warnings",
+  ]
 
   if (is_clang) {
     # For UD_ASSERT(!"message");