Enable the -Wundef warning for clang

webrtc/test/testsupport/frame_writer_unittest.cc

 /*
  *  Copyright (c) 2011 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/test/testsupport/frame_writer.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/test/testsupport/fileutils.h"
 
 namespace webrtc {
 namespace test {
 
 const size_t kFrameLength = 1000;
 
 class FrameWriterTest: public testing::Test {
  protected:
   FrameWriterTest() {}
@@ skipping 30 matching lines @@
 }
 
 TEST_F(FrameWriterTest, WriteFrameUninitialized) {
   uint8_t buffer[3];
   FrameWriterImpl frame_writer(temp_filename_, kFrameLength);
   ASSERT_FALSE(frame_writer.WriteFrame(buffer));
 }
 
 }  // namespace test
 }  // namespace webrtc