Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/differ_unittest.cc

 /*
  *  Copyright (c) 2013 The WebRTC 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 in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #include <memory>
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/base/gmock.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/modules/desktop_capture/differ.h"
 #include "webrtc/modules/desktop_capture/differ_block.h"
 
 namespace webrtc {
 
 // 96x96 screen gives a 4x4 grid of blocks.
 const int kScreenWidth= 96;
 const int kScreenHeight = 96;
 
@@ skipping 621 matching lines @@
 
   dirty.Clear();
   MergeBlocks(&dirty);
 
   ASSERT_EQ(2, RegionRectCount(dirty));
   ASSERT_TRUE(CheckDirtyRegionContainsRect(dirty, 0, 0, 2, 2));
   ASSERT_TRUE(CheckDirtyRegionContainsRect(dirty, 1, 2, 1, 1));
 }
 
 }  // namespace webrtc