Enable the -Wundef warning for clang

webrtc/modules/audio_device/ios/objc/RTCAudioSessionTest.mm

 /*
  *  Copyright 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.
  */
 
 #import <Foundation/Foundation.h>
 
-#include "testing/gtest/include/gtest/gtest.h"
+#include "webrtc/test/gtest.h"
 
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSession.h"
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSession+Private.h"
 
 @interface RTCAudioSessionTestDelegate : NSObject <RTCAudioSessionDelegate>
 @end
 
 @implementation RTCAudioSessionTestDelegate
 
 - (void)audioSessionDidBeginInterruption:(RTCAudioSession *)session {
@@ skipping 185 matching lines @@
   RTCAudioSessionTest *test = [[RTCAudioSessionTest alloc] init];
   [test testZeroingWeakDelegate];
 }
 
 TEST_F(AudioSessionTest, RemoveDelegateOnDealloc) {
   RTCAudioSessionTest *test = [[RTCAudioSessionTest alloc] init];
   [test testRemoveDelegateOnDealloc];
 }
 
 }  // namespace webrtc