linux and chromeos: Turn on -Wunused-const-variable.

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 3180 matching lines @@
               '-Wno-c++11-narrowing',
 
               # TODO(thakis): Remove, http://crbug.com/263960
               '-Wno-reserved-user-defined-literal',
 
               # Clang considers the `register` keyword as deprecated, but e.g.
               # code generated by flex (used in angle) contains that keyword.
               # http://crbug.com/255186
               '-Wno-deprecated-register',
 
-              # TODO(hans): Remove once we've cleaned up the warnings.
-              # http://crbug.com/307668
-              '-Wno-unused-const-variable',
-
               # This warns about auto_ptr<>, used in third-party code.
               '-Wno-deprecated-declarations',
             ],
             'cflags!': [
               # Clang doesn't seem to know know this flag.
               '-mfpmath=sse',
             ],
             'cflags_cc': [
               # See the comment in the Mac section for what it takes to move
               # this to -std=c++11.
               '-std=gnu++11',
             ],
           }],
           ['clang==1 and OS=="android"', {
             # Android uses stlport, whose include/new defines
             # `void  operator delete[](void* ptr) throw();`, which
             # clang's -Wimplicit-exception-spec-mismatch warns about for some
             # reason -- http://llvm.org/PR16638. TODO(thakis): Include stlport
             # via -isystem instead.
             'cflags_cc': [
               '-Wno-implicit-exception-spec-mismatch',
             ],
+            'cflags': [
+              # TODO(hans): Remove once we've cleaned up the warnings.
+              # http://crbug.com/307668
+              '-Wno-unused-const-variable',
+            ],
           }],
           ['clang==1 and clang_use_chrome_plugins==1', {
             'cflags': [
               '<@(clang_chrome_plugins_flags)',
             ],
           }],
           ['clang==1 and clang_load!=""', {
             'cflags': [
               '-Xclang', '-load', '-Xclang', '<(clang_load)',
             ],
@@ skipping 1490 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',
   },
 }