Use RTCAudioSessionDelegate in AudioDeviceIOS.

webrtc/modules/audio_device/ios/audio_session_observer.h

Index: webrtc/modules/audio_device/ios/audio_session_observer.h
diff --git a/webrtc/modules/audio_device/ios/audio_session_observer.h b/webrtc/modules/audio_device/ios/audio_session_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..d05e8c9a3e5e8698673108f72f9e2673b59b3c25
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+++ b/webrtc/modules/audio_device/ios/audio_session_observer.h
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+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_MODULES_AUDIO_DEVICE_IOS_AUDIO_SESSION_OBSERVER_H_
+#define WEBRTC_MODULES_AUDIO_DEVICE_IOS_AUDIO_SESSION_OBSERVER_H_
+
+#include "webrtc/base/asyncinvoker.h"
+#include "webrtc/base/thread.h"
+
+namespace webrtc {
+
+// Observer interface for listening to AVAudioSession events.
+class AudioSessionObserver {
+ public:
+  // Called when audio session interruption begins.
+  virtual void OnInterruptionBegin() = 0;
+
+  // Called when audio session interruption ends.
+  virtual void OnInterruptionEnd() = 0;
+
+  // Called when audio route changes.
+  virtual void OnValidRouteChange() = 0;
+
+ protected:
+  virtual ~AudioSessionObserver() {}
+};
+
+}  // namespace webrtc
+
+#endif  //  WEBRTC_MODULES_AUDIO_DEVICE_IOS_AUDIO_SESSION_OBSERVER_H_