Enable the -Wundef warning for clang

webrtc/video/screenshare_loopback.cc

 /*
  *  Copyright (c) 2015 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 <stdio.h>
 
 #include "gflags/gflags.h"
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/base/gtest.h"
 
 #include "webrtc/test/field_trial.h"
 #include "webrtc/test/run_test.h"
 #include "webrtc/video/video_quality_test.h"
 
 namespace webrtc {
 namespace flags {
 
 // Flags common with video loopback, with different default values.
 DEFINE_int32(width, 1850, "Video width (crops source).");
@@ skipping 234 matching lines @@
 }  // namespace webrtc
 
 int main(int argc, char* argv[]) {
   ::testing::InitGoogleTest(&argc, argv);
   google::ParseCommandLineFlags(&argc, &argv, true);
   webrtc::test::InitFieldTrialsFromString(
       webrtc::flags::FLAGS_force_fieldtrials);
   webrtc::test::RunTest(webrtc::Loopback);
   return 0;
 }