Enable the -Wundef warning for clang

webrtc/modules/video_coding/receiver_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 <string.h>
 
 #include <list>
 #include <memory>
 #include <queue>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/checks.h"
 #include "webrtc/modules/video_coding/encoded_frame.h"
 #include "webrtc/modules/video_coding/packet.h"
 #include "webrtc/modules/video_coding/receiver.h"
 #include "webrtc/modules/video_coding/test/stream_generator.h"
 #include "webrtc/modules/video_coding/timing.h"
 #include "webrtc/modules/video_coding/test/test_util.h"
 #include "webrtc/system_wrappers/include/clock.h"
 #include "webrtc/system_wrappers/include/critical_section_wrapper.h"
 
@@ skipping 471 matching lines @@
                 end_time);
       receiver_.ReleaseFrame(frame);
       ++num_frames_return;
     } else {
       EXPECT_EQ(kMaxWaitTime, end_time - start_time);
     }
   }
 }
 
 }  // namespace webrtc