Enable the -Wundef warning for clang

webrtc/test/testsupport/perf_test_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 "webrtc/test/testsupport/perf_test.h"
 
 #include <string>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 namespace webrtc {
 namespace test {
 
 TEST(PerfTest, AppendResult) {
   std::string expected = "RESULT measurementmodifier: trace= 42 units\n";
   std::string output;
   AppendResult(output, "measurement", "modifier", "trace", 42, "units", false);
   EXPECT_EQ(expected, output);
   std::cout << output;
 
   expected += "*RESULT foobar: baz= 7 widgets\n";
   AppendResult(output, "foo", "bar", "baz", 7, "widgets", true);
   EXPECT_EQ(expected, output);
   std::cout << output;
 }
 
 }  // namespace test
 }  // namespace webrtc