Enable clang warning for ignored attributes.

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")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 784 matching lines @@
   # Building with Clang on Windows is a work in progress and very
   # experimental. See crbug.com/82385.
   # Keep this in sync with the similar block in build/common.gypi
   if (is_clang) {
     default_warning_flags += [
       # TODO(hans): Make this list shorter eventually, http://crbug.com/504657
       "-Qunused-arguments",  # http://crbug.com/504658
       "-Wno-c++11-compat-deprecated-writable-strings",  # http://crbug.com/504659
       "-Wno-empty-body",  # http://crbug.com/504661
       "-Wno-extra-tokens",  # http://crbug.com/504663
-      "-Wno-ignored-attributes",  # http://crbug.com/504695
       "-Wno-incompatible-pointer-types",  # http://crbug.com/504696
       "-Wno-int-to-void-pointer-cast",  # http://crbug.com/504697
       "-Wno-invalid-noreturn",  # http://crbug.com/504698
       "-Wno-logical-op-parentheses",  # http://crbug.com/504699
       "-Wno-microsoft",  # http://crbug.com/505296
       "-Wno-missing-braces",  # http://crbug.com/505297
       "-Wno-msvc-include",  # http://crbug.com/505299
       "-Wno-overloaded-virtual",  # http://crbug.com/505301
       "-Wno-pointer-sign",  # http://crbug.com/505303
       "-Wno-reorder",  # http://crbug.com/505304
@@ skipping 421 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }