Use RTCAudioSessionDelegate in AudioDeviceIOS.

webrtc/modules/audio_device/ios/objc/RTCAudioSessionDelegateAdapter.h

+/*
+ *  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 "webrtc/modules/audio_device/ios/objc/RTCAudioSession.h"
+
+namespace webrtc {
+  class AudioDeviceIOS;
+}
+
+namespace rtc {
+  class Thread;
+}
+
+/** Adapter that forwards RTCAudioSessionDelegate calls to the appropriate
+ *  methods on the audio device. In addition, calls those methods async on the
+ *  thread supplied during construction.
+ */
+@interface RTCAudioSessionDelegateAdapter : NSObject <RTCAudioSessionDelegate>
+
+- (instancetype)init NS_UNAVAILABLE;
+
+/** Audio device and callback thread are raw pointers and should be kept alive

[henrika_webrtc, 2016/03/15 08:53:44]

Are there any restrictions on the thread here in our design? Could it be any?
Should prio be high, low etc.?
Just curious if any details should be added as comment here.

[tkchin_webrtc, 2016/03/15 20:14:58]

Good pt. Edited interfaces a bit.

+ *  for this object's lifetime.
+ */
+- (instancetype)initWithAudioDevice:(webrtc::AudioDeviceIOS *)device
+                     callbackThread:(rtc::Thread *)callbackThread
+    NS_DESIGNATED_INITIALIZER;
+
+@end