Enable the -Wundef warning for clang

webrtc/modules/desktop_capture/screen_capturer_helper_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 "webrtc/modules/desktop_capture/screen_capturer_helper.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 
 class ScreenCapturerHelperTest : public testing::Test {
  protected:
   ScreenCapturerHelper capturer_helper_;
 };
 
 TEST_F(ScreenCapturerHelperTest, ClearInvalidRegion) {
   DesktopRegion region(DesktopRect::MakeXYWH(1, 2, 3, 4));
@@ skipping 154 matching lines @@
               x, y, kGridSize, kGridSize));
         }
 
         TestExpandRegionToGrid(region, kLogGridSize, expanded_region_expected);
       }
     }
   }
 }
 
 }  // namespace webrtc