Enable the -Wundef warning for clang

webrtc/base/rate_limiter_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 <algorithm>
 #include <memory>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/base/event.h"
 #include "webrtc/base/platform_thread.h"
 #include "webrtc/base/rate_limiter.h"
 #include "webrtc/base/task_queue.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 
 class RateLimitTest : public ::testing::Test {
@@ skipping 171 matching lines @@
 
   // All rate consumed.
   EXPECT_FALSE(rate_limiter->TryUseRate(1));
 
   thread1.Stop();
   thread2.Stop();
   thread3.Stop();
 }
 
 }  // namespace webrtc