Enable the -Wundef warning for clang

webrtc/api/proxy_unittest.cc

 /*
  *  Copyright 2013 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/api/proxy.h"
 
 #include <memory>
 #include <string>
 
-#include "testing/gmock/include/gmock/gmock.h"
+#include "webrtc/test/gmock.h"
 #include "webrtc/base/gunit.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/base/thread.h"
 
 using ::testing::_;
 using ::testing::DoAll;
 using ::testing::Exactly;
 using ::testing::InvokeWithoutArgs;
 using ::testing::Return;
 
@@ skipping 203 matching lines @@
   const std::string arg1 = "arg1";
   const std::string arg2 = "arg2";
   EXPECT_CALL(*fake_, Method2(arg1, arg2))
       .Times(Exactly(1))
       .WillOnce(DoAll(InvokeWithoutArgs(this, &ProxyTest::CheckWorkerThread),
                       Return("Method2")));
   EXPECT_EQ("Method2", fake_proxy_->Method2(arg1, arg2));
 }
 
 }  // namespace webrtc