Enable the -Wundef warning for clang

webrtc/modules/video_coding/jitter_estimator_tests.cc

 /*  Copyright (c) 2014 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/video_coding/jitter_estimator.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 #include "webrtc/system_wrappers/include/clock.h"
 
 namespace webrtc {
 
 class TestEstimator : public VCMJitterEstimator {
  public:
   explicit TestEstimator(bool exp_enabled)
       : VCMJitterEstimator(&fake_clock_, 0, 0),
         fake_clock_(0),
         exp_enabled_(exp_enabled) {}
@@ skipping 128 matching lines @@
     }
 
     gen.Advance();
   }
 
   EXPECT_NE(regular_iterations, 0);
   EXPECT_NE(low_rate_iterations, 0);
   EXPECT_LE(low_rate_iterations, regular_iterations);
 }
 }