Enable the -Wundef warning for clang

webrtc/test/gmock.h

 /*
  *  Copyright (c) 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.
  */
 
-#include "webrtc/modules/audio_coding/neteq/packet.h"
+#ifndef WEBRTC_TEST_GMOCK_H_
+#define WEBRTC_TEST_GMOCK_H_
 
-namespace webrtc {
+#include "webrtc/base/ignore_wundef.h"
 
-Packet::Packet() = default;
+RTC_PUSH_IGNORING_WUNDEF()
+#include "testing/gmock/include/gmock/gmock.h"
+RTC_POP_IGNORING_WUNDEF()
 
-Packet::~Packet() = default;
-
-}  // namespace webrtc
+#endif  // WEBRTC_TEST_GMOCK_H_