Enable the -Wundef warning for clang

webrtc/common_video/i420_buffer_pool_unittest.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 <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/common_video/include/i420_buffer_pool.h"
 
 namespace webrtc {
 
 TEST(TestI420BufferPool, SimpleFrameReuse) {
   I420BufferPool pool;
   rtc::scoped_refptr<VideoFrameBuffer> buffer = pool.CreateBuffer(16, 16);
   EXPECT_EQ(16, buffer->width());
   EXPECT_EQ(16, buffer->height());
   // Extract non-refcounted pointers for testing.
@@ skipping 33 matching lines @@
     I420BufferPool pool;
     buffer = pool.CreateBuffer(16, 16);
   }
   EXPECT_EQ(16, buffer->width());
   EXPECT_EQ(16, buffer->height());
   // Try to trigger use-after-free errors by writing to y-plane.
   memset(buffer->MutableDataY(), 0xA5, 16 * buffer->StrideY());
 }
 
 }  // namespace webrtc