Use RTCAudioSessionDelegate in AudioDeviceIOS.

webrtc/modules/audio_device/audio_device.gypi

 # Copyright (c) 2012 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.
 
 {
   'targets': [
@@ skipping 155 matching lines @@
               },
             }],
             ['OS=="ios"', {
               'dependencies': [
                 '<(webrtc_root)/base/base.gyp: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',
               ],
               'xcode_settings': {
                 'CLANG_ENABLE_OBJC_ARC': 'YES',
               },
               'link_settings': {
                 'xcode_settings': {
                   'OTHER_LDFLAGS': [
                     '-framework AudioToolbox',
                     '-framework AVFoundation',
                     '-framework Foundation',
@@ skipping 82 matching lines @@
             'test/audio_device_test_defines.h',
             'test/func_test_manager.cc',
             'test/func_test_manager.h',
           ],
         },
       ], # targets
     }], # include_tests==1 and OS!=ios
   ],
 }