Fix lock behavior on RTCAudioSession.

webrtc/base/scoped_autorelease_pool.mm

 /*
  *  Copyright 2008 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.
  */
 
+#if defined(WEBRTC_MAC)
+
 #import <Foundation/Foundation.h>
 
 #import "webrtc/base/scoped_autorelease_pool.h"
 
 namespace rtc {
 
 ScopedAutoreleasePool::ScopedAutoreleasePool() {
   pool_ = [[NSAutoreleasePool alloc] init];
 }
 
 ScopedAutoreleasePool::~ScopedAutoreleasePool() {
   [pool_ drain];
 }
 
 }  // namespace rtc
+
+#endif  // defined(WERTC_MAC)