Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtcp_packet/common_header_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 "webrtc/modules/rtp_rtcp/source/rtcp_packet/common_header.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 using webrtc::rtcp::CommonHeader;
 
 namespace webrtc {
 namespace {
 const size_t kHeaderSizeBytes = 4;
 }  // namespace
 
 TEST(RtcpCommonHeaderTest, TooSmallBuffer) {
   uint8_t buffer[] = {0x80, 0x00, 0x00, 0x00};
@@ skipping 71 matching lines @@
   CommonHeader header;
   EXPECT_TRUE(header.Parse(buffer, sizeof(buffer)));
 
   EXPECT_EQ(header.count(), 0x1e);
   EXPECT_EQ(header.fmt(), 0x1e);
   EXPECT_EQ(header.type(), 0xab);
   EXPECT_EQ(header.payload_size_bytes(), 0u);
   EXPECT_EQ(header.payload(), buffer + kHeaderSizeBytes);
 }
 }  // namespace webrtc