Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/screen_drawer_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_drawer.h"
 
 #include <stdint.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/base/random.h"
 #include "webrtc/base/timeutils.h"
 #include "webrtc/system_wrappers/include/logging.h"
 #include "webrtc/system_wrappers/include/sleep.h"
 
 namespace webrtc {
 
 // These are a set of manual test cases, as we do not have an automatical way to
 // detect whether a ScreenDrawer on a certain platform works well without
 // ScreenCapturer(s). So you may execute these test cases with
@@ skipping 25 matching lines @@
 
     if (i == 50) {
       SleepMs(10000);
     }
   }
 
   SleepMs(10000);
 }
 
 }  // namespace webrtc