Improve iOS frame capture threading.

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"
 
+#import "RTCDispatcher+Private.h"
+
 static dispatch_queue_t kAudioSessionQueue = nil;
 static dispatch_queue_t kCaptureSessionQueue = nil;
 
 @implementation RTCDispatcher
 
 + (void)initialize {
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
     kAudioSessionQueue = dispatch_queue_create(
         "org.webrtc.RTCDispatcherAudioSession",
@@ skipping 18 matching lines @@
       return dispatch_get_main_queue();
     case RTCDispatcherTypeCaptureSession:
       return kCaptureSessionQueue;
     case RTCDispatcherTypeAudioSession:
       return kAudioSessionQueue;
   }
 }
 
 @end