Enable the -Wundef warning for clang

webrtc/media/engine/payload_type_mapper_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 <set>
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/media/engine/payload_type_mapper.h"
 
 namespace cricket {
 
 class PayloadTypeMapperTest : public testing::Test {
  public:
   // TODO(ossu): These are to work around missing comparison operators in
   // rtc::Optional. They should be removed once Optional has been updated.
   int FindMapping(const webrtc::SdpAudioFormat& format) {
     auto opt_mapping = mapper_.FindMappingFor(format);
@@ skipping 128 matching lines @@
 
     EXPECT_EQ(codec.id, payload_type);
     EXPECT_EQ(codec.name, format.name);
     EXPECT_EQ(codec.clockrate, format.clockrate_hz);
     EXPECT_EQ(codec.channels, format.num_channels);
     EXPECT_EQ(codec.params, format.parameters);
   }
 }
 
 }  // namespace cricket