Enable the -Wundef warning for clang

webrtc/modules/video_coding/session_info_unittest.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 <string.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/modules/video_coding/packet.h"
 #include "webrtc/modules/video_coding/session_info.h"
 
 namespace webrtc {
 
 class TestSessionInfo : public ::testing::Test {
  protected:
   virtual void SetUp() {
     memset(packet_buffer_, 0, sizeof(packet_buffer_));
@@ skipping 997 matching lines @@
   FillPacket(1);
   EXPECT_EQ(packet_buffer_size(),
             static_cast<size_t>(session_.InsertPacket(packet_, frame_buffer_,
                                                       kNoErrors, frame_data)));
 
   EXPECT_EQ(2 * packet_buffer_size(), session_.MakeDecodable());
   EXPECT_EQ(0U, session_.SessionLength());
 }
 
 }  // namespace webrtc