Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/screen_capturer_mock_objects.h

 /*
  *  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.
  */
 
 #ifndef WEBRTC_MODULES_DESKTOP_CAPTURE_SCREEN_CAPTURER_MOCK_OBJECTS_H_
 #define WEBRTC_MODULES_DESKTOP_CAPTURE_SCREEN_CAPTURER_MOCK_OBJECTS_H_
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/modules/desktop_capture/screen_capturer.h"
 
 namespace webrtc {
 
 class MockScreenCapturer : public ScreenCapturer {
  public:
   MockScreenCapturer() {}
   virtual ~MockScreenCapturer() {}
 
@@ skipping 19 matching lines @@
     OnCaptureResultPtr(result, &frame);
   }
 
  private:
   RTC_DISALLOW_COPY_AND_ASSIGN(MockScreenCapturerCallback);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_SCREEN_CAPTURER_MOCK_OBJECTS_H_