Use RTCAudioSessionDelegate in AudioDeviceIOS.

webrtc/modules/audio_device/BUILD.gn

 # Copyright (c) 2014 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("../../build/webrtc.gni")
 
@@ skipping 117 matching lines @@
         "AudioToolbox.framework",
         "CoreAudio.framework",
       ]
     }
     if (is_ios) {
       deps += [ "../../base:rtc_base_objc" ]
       sources += [
         "ios/audio_device_ios.h",
         "ios/audio_device_ios.mm",
         "ios/audio_device_not_implemented_ios.mm",
+        "ios/audio_session_observer.h",
         "ios/objc/RTCAudioSession+Configuration.mm",
         "ios/objc/RTCAudioSession+Private.h",
         "ios/objc/RTCAudioSession.h",
         "ios/objc/RTCAudioSession.mm",
         "ios/objc/RTCAudioSessionConfiguration.h",
         "ios/objc/RTCAudioSessionConfiguration.m",
+        "ios/objc/RTCAudioSessionDelegateAdapter.h",
+        "ios/objc/RTCAudioSessionDelegateAdapter.mm",
       ]
       cflags += [ "-fobjc-arc" ]  # CLANG_ENABLE_OBJC_ARC = YES.
       libs = [
         "AudioToolbox.framework",
         "AVFoundation.framework",
         "Foundation.framework",
         "UIKit.framework",
       ]
     }
     if (is_win) {
@@ skipping 30 matching lines @@
     "../..:common_inherited_config",
     ":audio_device_config",
   ]
 
   if (is_clang) {
     # Suppress warnings from Chrome's Clang plugins.
     # See http://code.google.com/p/webrtc/issues/detail?id=163 for details.
     configs -= [ "//build/config/clang:find_bad_constructs" ]
   }
 }