Fix lock behavior on RTCAudioSession.

webrtc/base/objc/RTCDispatcher.m

 /*
  *  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 "RTCDispatcher.h"
 
+static dispatch_queue_t kAudioSessionQueue = nil;
 static dispatch_queue_t kCaptureSessionQueue = nil;
 
-@implementation RTCDispatcher {
-  dispatch_queue_t _captureSessionQueue;
-}
+@implementation RTCDispatcher
 
 + (void)initialize {
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
+    kAudioSessionQueue = dispatch_queue_create(
+        "org.webrtc.RTCDispatcherAudioSession",
+        DISPATCH_QUEUE_SERIAL);
     kCaptureSessionQueue = dispatch_queue_create(
         "org.webrtc.RTCDispatcherCaptureSession",
         DISPATCH_QUEUE_SERIAL);
   });
 }
 
 + (void)dispatchAsyncOnType:(RTCDispatcherQueueType)dispatchType
                       block:(dispatch_block_t)block {
   dispatch_queue_t queue = [self dispatchQueueForType:dispatchType];
   dispatch_async(queue, block);
 }
 
 #pragma mark - Private
 
 + (dispatch_queue_t)dispatchQueueForType:(RTCDispatcherQueueType)dispatchType {
   switch (dispatchType) {
     case RTCDispatcherTypeMain:
       return dispatch_get_main_queue();
     case RTCDispatcherTypeCaptureSession:
       return kCaptureSessionQueue;
+    case RTCDispatcherTypeAudioSession:
+      return kAudioSessionQueue;
   }
 }
 
 @end