Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/test/testAPI/test_api_video.cc

 /*
  *  Copyright (c) 2012 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 <stdlib.h>
 
 #include <algorithm>
 #include <memory>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/rate_limiter.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_payload_registry.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/modules/rtp_rtcp/source/byte_io.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_receiver_video.h"
 #include "webrtc/modules/rtp_rtcp/test/testAPI/test_api.h"
 
 namespace {
@@ skipping 156 matching lines @@
                                                    payload_specific, true));
       EXPECT_EQ(0u, receiver_->payload_size());
       EXPECT_EQ(payload_length, receiver_->rtp_header().header.paddingLength);
     }
     timestamp += 3000;
     fake_clock.AdvanceTimeMilliseconds(33);
   }
 }
 
 }  // namespace webrtc