Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/window_capturer_unittest.cc

 /*
  *  Copyright (c) 2013 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 <memory>
 
 #include "webrtc/modules/desktop_capture/window_capturer.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/modules/desktop_capture/desktop_capture_options.h"
 #include "webrtc/modules/desktop_capture/desktop_frame.h"
 #include "webrtc/modules/desktop_capture/desktop_region.h"
 #include "webrtc/system_wrappers/include/logging.h"
 
 namespace webrtc {
 
 class WindowCapturerTest : public testing::Test,
                            public DesktopCapturer::Callback {
  public:
@@ skipping 57 matching lines @@
       }
       continue;
     }
 
     EXPECT_GT(frame_->size().width(), 0);
     EXPECT_GT(frame_->size().height(), 0);
   }
 }
 
 }  // namespace webrtc