Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/screen_capturer_differ_wrapper_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 "webrtc/modules/desktop_capture/screen_capturer_differ_wrapper.h"
 
 #include <initializer_list>
 #include <memory>
 #include <utility>
 #include <vector>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/base/timeutils.h"
 #include "webrtc/modules/desktop_capture/desktop_geometry.h"
 #include "webrtc/modules/desktop_capture/desktop_region.h"
 #include "webrtc/modules/desktop_capture/differ_block.h"
 #include "webrtc/modules/desktop_capture/fake_screen_capturer.h"
 #include "webrtc/modules/desktop_capture/screen_capturer_mock_objects.h"
 #include "webrtc/system_wrappers/include/cpu_features_wrapper.h"
 #include "webrtc/typedefs.h"
 
@@ skipping 253 matching lines @@
 }
 
 TEST(ScreenCapturerDifferWrapperTest,
      DISABLED_CaptureWithEnlargedAndRandomHintsPerf) {
   int64_t started = rtc::TimeMillis();
   ExecuteDifferWrapperTest(true, true, true, false);
   ASSERT_LE(rtc::TimeMillis() - started, 15000);
 }
 
 }  // namespace webrtc