Enable the -Wundef warning for clang

webrtc/modules/video_coding/decoding_state_unittest.cc

 /*
  *  Copyright (c) 2011 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/decoding_state.h"
 #include "webrtc/modules/video_coding/frame_buffer.h"
 #include "webrtc/modules/video_coding/jitter_buffer_common.h"
 #include "webrtc/modules/video_coding/packet.h"
 
 namespace webrtc {
 
 TEST(TestDecodingState, Sanity) {
   VCMDecodingState dec_state;
@@ skipping 666 matching lines @@
   // Ref to frame previous to KF, not continuous
   frame.Reset();
   vp9_hdr.picture_id = 30;
   vp9_hdr.num_ref_pics = 1;
   vp9_hdr.pid_diff[0] = 30;
   EXPECT_LE(0, frame.InsertPacket(packet, 0, kNoErrors, frame_data));
   EXPECT_FALSE(dec_state.ContinuousFrame(&frame));
 }
 
 }  // namespace webrtc