Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtp_header_extension_unittest.cc

 /*
  *  Copyright (c) 2012 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 "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_header_extension.h"
 #include "webrtc/typedefs.h"
 
 namespace webrtc {
 
 class RtpHeaderExtensionTest : public ::testing::Test {
  protected:
   RtpHeaderExtensionTest() {}
@@ skipping 103 matching lines @@
   EXPECT_EQ(kRtpExtensionTransmissionTimeOffset, mapOut.First());
 }
 
 TEST_F(RtpHeaderExtensionTest, Erase) {
   EXPECT_EQ(0, map_.Register(kRtpExtensionTransmissionTimeOffset, kId));
   EXPECT_EQ(1, map_.Size());
   map_.Erase();
   EXPECT_EQ(0, map_.Size());
 }
 }  // namespace webrtc