Move audio focus control from media/ to content/ and make it per WebContents.

content/browser/media/android/media_session_observer.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_BROWSER_MEDIA_ANDROID_MEDIA_SESSION_OBSERVER_H_
+#define CONTENT_BROWSER_MEDIA_ANDROID_MEDIA_SESSION_OBSERVER_H_
+
+namespace content {
+
+class MediaSessionObserver {
+ public:
+  MediaSessionObserver() = default;
+  virtual ~MediaSessionObserver() = default;
+
+  // The given |player_id| has been suspended by the MediaSession.
+  virtual void OnSuspend(int player_id) = 0;
+
+  // The given |player_id| has been resumed by the MediaSession.
+  virtual void OnResume(int player_id) = 0;
+};
+
+}  // namespace content
+
+#endif // CONTENT_BROWSER_MEDIA_ANDROID_MEDIA_SESSION_OBSERVER_H_