Linux/CrOS/Cast/Android (and Mac with gn): Build chromium_code with -Wextra.

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/ccache.gni")
 
@@ skipping 876 matching lines @@
 
 # chromium_code ---------------------------------------------------------------
 #
 # Toggles between higher and lower warnings for code that is (or isn't)
 # part of Chromium.
 
 config("chromium_code") {
   if (is_win) {
     cflags = [ "/W4" ]  # Warning level 4.
   } else {
-    cflags = [
-      "-Wall",
-
-      # GCC turns on -Wsign-compare for C++ under -Wall, but clang doesn't,
-      # so we specify it explicitly.
-      # TODO(fischman): remove this if http://llvm.org/PR10448 obsoletes it.
-      # http://code.google.com/p/chromium/issues/detail?id=90453
-      "-Wsign-compare",
-    ]
+    cflags = [ "-Wall" ]
+    if (is_clang) {
+      # Enable -Wextra for chromium_code when we control the compiler.
+      cflags += [ "-Wextra" ]
+    }
 
     # In Chromium code, we define __STDC_foo_MACROS in order to get the
     # C99 macros on Mac and Linux.
     defines = [
       "__STDC_CONSTANT_MACROS",
       "__STDC_FORMAT_MACROS",
     ]
 
     if (!is_debug && !using_sanitizer &&
         (!is_linux || !is_clang || is_official_build)) {
@@ skipping 391 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }