Enable the -Wundef warning for clang

webrtc/video/vie_encoder_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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/base/gtest.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/test/encoder_settings.h"
 #include "webrtc/test/fake_encoder.h"
 #include "webrtc/test/frame_generator.h"
 #include "webrtc/video/send_statistics_proxy.h"
 #include "webrtc/video/vie_encoder.h"
 
 namespace webrtc {
 
 class ViEEncoderTest : public ::testing::Test {
@@ skipping 227 matching lines @@
   // call to ContinueEncode.
   video_source_.IncomingCapturedFrame(CreateFrame(2, nullptr));
   video_source_.IncomingCapturedFrame(CreateFrame(3, nullptr));
   fake_encoder_.ContinueEncode();
   sink_.WaitForEncodedFrame(3);
 
   vie_encoder_->Stop();
 }
 
 }  // namespace webrtc