Enable the -Wundef warning for clang

webrtc/modules/video_coding/codecs/vp8/simulcast_encoder_adapter_unittest.cc

 /*
  *  Copyright (c) 2014 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 <vector>
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/base/gmock.h"
 #include "webrtc/modules/video_coding/include/video_codec_interface.h"
 #include "webrtc/modules/video_coding/codecs/vp8/simulcast_encoder_adapter.h"
 #include "webrtc/modules/video_coding/codecs/vp8/simulcast_unittest.h"
 
 namespace webrtc {
 namespace testing {
 
 class TestSimulcastEncoderAdapter : public TestVp8Simulcast {
  public:
   TestSimulcastEncoderAdapter()
@@ skipping 521 matching lines @@
   memset(input_frame.video_frame_buffer()->MutableDataV(), 0,
          input_frame.allocated_size(kVPlane));
 
   std::vector<FrameType> frame_types(3, kVideoFrameKey);
   EXPECT_EQ(WEBRTC_VIDEO_CODEC_FALLBACK_SOFTWARE,
             adapter_->Encode(input_frame, nullptr, &frame_types));
 }
 
 }  // namespace testing
 }  // namespace webrtc