Fix lock behavior on RTCAudioSession.

webrtc/base/objc/RTCDispatcher.h

 /*
  *  Copyright 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.
  */
 
 #import <Foundation/Foundation.h>
 
 typedef NS_ENUM(NSInteger, RTCDispatcherQueueType) {
   // Main dispatcher queue.
   RTCDispatcherTypeMain,
   // Used for starting/stopping AVCaptureSession, and assigning
   // capture session to AVCaptureVideoPreviewLayer.
   RTCDispatcherTypeCaptureSession,
+  // Used for operations on AVAudioSession.
+  RTCDispatcherTypeAudioSession,
 };
 
 /** Dispatcher that asynchronously dispatches blocks to a specific
  *  shared dispatch queue.
  */
 @interface RTCDispatcher : NSObject
 
 - (instancetype)init NS_UNAVAILABLE;
 
 /** Dispatch the block asynchronously on the queue for dispatchType.
  *  @param dispatchType The queue type to dispatch on.
  *  @param block The block to dispatch asynchronously.
  */
 + (void)dispatchAsyncOnType:(RTCDispatcherQueueType)dispatchType
                       block:(dispatch_block_t)block;
 
 @end