Use RTCAudioSessionDelegate in AudioDeviceIOS.

webrtc/modules/audio_device/ios/objc/RTCAudioSession+Private.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"
 
+#include <vector>
+
 NS_ASSUME_NONNULL_BEGIN
 
 @interface RTCAudioSession ()
 
-/** The delegates. */
-@property(nonatomic, readonly) NSSet *delegates;
-
 /** Number of times setActive:YES has succeeded without a balanced call to
  *  setActive:NO.
  */
 @property(nonatomic, readonly) NSInteger activationCount;
 
 - (BOOL)checkLock:(NSError **)outError;
 
+/** Adds the delegate to the list of delegates, and places it at the front of
+ *  the list. This delegate will be notified before other delegates of
+ *  audio events.
+ */
+- (void)pushDelegate:(id<RTCAudioSessionDelegate>)delegate;
+
+// Properties and methods for tests.
+@property(nonatomic, readonly)
+    std::vector<__weak id<RTCAudioSessionDelegate> > delegates;
+- (void)notifyDidBeginInterruption;
+- (void)notifyDidEndInterruptionWithShouldResumeSession:
+    (BOOL)shouldResumeSession;
+- (void)notifyDidChangeRouteWithReason:(AVAudioSessionRouteChangeReason)reason
+    previousRoute:(AVAudioSessionRouteDescription *)previousRoute;
+- (void)notifyMediaServicesWereLost;
+- (void)notifyMediaServicesWereReset;
+
 @end
 
 NS_ASSUME_NONNULL_END