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"
 
-NS_ASSUME_NONNULL_BEGIN
+namespace webrtc {
+class AudioSessionObserver;
+}
 
-@interface RTCAudioSession ()
+/** Adapter that forwards RTCAudioSessionDelegate calls to the appropriate
+ *  methods on the AudioSessionObserver.
+ */
+@interface RTCAudioSessionDelegateAdapter : NSObject <RTCAudioSessionDelegate>
 
-/** The delegates. */
-@property(nonatomic, readonly) NSSet *delegates;
+- (instancetype)init NS_UNAVAILABLE;
 
-/** Number of times setActive:YES has succeeded without a balanced call to
- *  setActive:NO.
+/** |observer| is a raw pointer and should be kept alive
+ *  for this object's lifetime.
  */
-@property(nonatomic, readonly) NSInteger activationCount;
-
-- (BOOL)checkLock:(NSError **)outError;
+- (instancetype)initWithObserver:(webrtc::AudioSessionObserver *)observer
+    NS_DESIGNATED_INITIALIZER;
 
 @end
-
-NS_ASSUME_NONNULL_END