Remove the intent to turn on Wextra for gcc (except MacOS)

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 3585 matching lines @@
           'libraries_for_target%': '',
         },
         'defines': [
           '_FILE_OFFSET_BITS=64',
         ],
         'cflags': [
           '<(werror)',  # See note above about the werror variable.
           '-pthread',
           '-fno-strict-aliasing',  # See http://crbug.com/32204
           '-Wall',
-          # TODO(evan): turn this back on once all the builds work.
-          # '-Wextra',
           # Don't warn about unused function params.  We use those everywhere.
           '-Wno-unused-parameter',
           # Don't warn about the "struct foo f = {0};" initialization pattern.
           '-Wno-missing-field-initializers',
           # Don't export any symbols (for example, to plugins we dlopen()).
           # Note: this is *required* to make some plugins work.
           '-fvisibility=hidden',
           '-pipe',
         ],
         'cflags_cc': [
@@ skipping 2475 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }