Fix x64 android_tools target compile failure.

build/android/ndk.gyp

 # Copyright (c) 2015 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.
 
 {
   'targets': [
     {
       'target_name': 'cpu_features',
       'type': 'static_library',
       'direct_dependent_settings': {
         'include_dirs': [
           '<(android_ndk_root)/sources/android/cpufeatures',
         ],
       },
       'sources': [
         '<(android_ndk_root)/sources/android/cpufeatures/cpu-features.c',
       ],
+      'variables': {
+        'clang_warning_flags': [
+          # cpu-features.c has few unused functions on x86 b/26403333
+          '-Wno-unused-function',
+        ],
+      },
     },
   ],
 }