Enable the -Wundef warning for clang

webrtc/api/test/mock_datachannel.h

 /*
  *  Copyright 2016 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_API_TEST_MOCK_DATACHANNEL_H_
 #define WEBRTC_API_TEST_MOCK_DATACHANNEL_H_
 
 #include "webrtc/api/datachannel.h"
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 
 namespace webrtc {
 
 class MockDataChannel : public rtc::RefCountedObject<DataChannel> {
  public:
   explicit MockDataChannel(DataState state)
       : rtc::RefCountedObject<DataChannel>(
             nullptr, cricket::DCT_NONE, "MockDataChannel") {
     EXPECT_CALL(*this, state()).WillRepeatedly(testing::Return(state));
   }
   MOCK_CONST_METHOD0(state, DataState());
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_TEST_MOCK_DATACHANNEL_H_