Enable the -Wundef warning for clang

webrtc/modules/video_coding/video_packet_buffer_unittest.cc

 /*
  *  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 <cstring>
 #include <map>
 #include <set>
 #include <utility>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/modules/video_coding/frame_object.h"
 #include "webrtc/modules/video_coding/packet_buffer.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 namespace video_coding {
 
 class TestPacketBuffer : public ::testing::Test,
                          public OnReceivedFrameCallback {
@@ skipping 323 matching lines @@
   packet.seqNum = Rand();
   EXPECT_TRUE(packet_buffer_->InsertPacket(packet));
   ASSERT_EQ(1UL, frames_from_callback_.size());
 
   packet_buffer_->Clear();
   EXPECT_FALSE(frames_from_callback_.begin()->second->GetBitstream(nullptr));
 }
 
 }  // namespace video_coding
 }  // namespace webrtc