Support clang-cl build on Windows.

testing/gtest.gyp

 # Copyright 2014 PDFium authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # Original code from V8, original license was:
 # Copyright 2014 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # This file is used only by the standalone PDFium build.  Under a chromium
@@ skipping 124 matching lines @@
               ['OS=="win"', {
                 'run_as': {
                   'action????': ['$(TargetPath)', '--gtest_print_time'],
                 },
               }],
             ],
           }],
         ],
         'msvs_disabled_warnings': [4800],
       },
+      'variables': {
+        'clang_warning_flags': [
+          # The Mutex constructor initializer list in gtest-port.cc is
+          # incorrectly ordered. See
+          # https://groups.google.com/d/msg/googletestframework/S5uSV8L2TX8/U1FaTDa6J6sJ.
+          '-Wno-reorder',
+          # Suppress warnings for unused constants.
+          '-Wno-unused'
+        ],
+      },
     },
     {
       'target_name': 'gtest_main',
       'type': 'static_library',
       'dependencies': [
         'gtest',
       ],
       'sources': [
         'gtest/src/gtest_main.cc',
       ],
     },
     {
       'target_name': 'gtest_prod',
       'toolsets': ['host', 'target'],
       'type': 'none',
       'sources': [
         'gtest/include/gtest/gtest_prod.h',
       ],
     },
   ],
 }