Enable the -Wundef warning for clang

webrtc/modules/rtp_rtcp/source/rtcp_packet/rrtr_unittest.cc

 /*
  *  Copyright (c) 2015 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/rrtr.h"
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 using webrtc::rtcp::Rrtr;
 
 namespace webrtc {
 namespace {
 
 const uint32_t kNtpSec = 0x12345678;
 const uint32_t kNtpFrac = 0x23456789;
 const uint8_t kBlock[] = {0x04, 0x00, 0x00, 0x02,
                           0x12, 0x34, 0x56, 0x78,
@@ skipping 18 matching lines @@
 
   // Run checks on const object to ensure all accessors have const modifier.
   const Rrtr& parsed = read_rrtr;
 
   EXPECT_EQ(kNtpSec, parsed.ntp().seconds());
   EXPECT_EQ(kNtpFrac, parsed.ntp().fractions());
 }
 
 }  // namespace
 }  // namespace webrtc