Enable the -Wundef warning for clang

webrtc/api/test/mock_peerconnection.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_PEERCONNECTION_H_
 #define WEBRTC_API_TEST_MOCK_PEERCONNECTION_H_
 
 #include <vector>
 
 #include "webrtc/api/peerconnection.h"
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 
 namespace webrtc {
 
 // The factory isn't really used; it just satisfies the base PeerConnection.
 class FakePeerConnectionFactory
     : public rtc::RefCountedObject<webrtc::PeerConnectionFactory> {};
 
 class MockPeerConnection
     : public rtc::RefCountedObject<webrtc::PeerConnection> {
  public:
   MockPeerConnection()
       : rtc::RefCountedObject<webrtc::PeerConnection>(
             new FakePeerConnectionFactory()) {}
   MOCK_METHOD0(session, WebRtcSession*());
   MOCK_CONST_METHOD0(sctp_data_channels,
                      const std::vector<rtc::scoped_refptr<DataChannel>>&());
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_TEST_MOCK_PEERCONNECTION_H_