Make MediaSession a runtime-enabled feature on Desktop.

content/browser/media/session/media_session_delegate.h

+// Copyright 2016 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_SESSION_MEDIA_SESSION_DELEGATE_H_
+#define CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_DELEGATE_H_
+
+#include "content/browser/media/session/media_session.h"
+
+namespace content {
+
+// MediaSessionDelegate is an interface abstracting audio focus handling for the
+// MediaSession class.
+class MediaSessionDelegate {
+ public:
+  // Factory method returning an implementation of MediaSessionDelegate.
+  static scoped_ptr<MediaSessionDelegate> Create(MediaSession* media_session);
+
+  virtual bool RequestAudioFocus(MediaSession::Type type) = 0;
+  virtual void AbandonAudioFocus() = 0;
+};
+
+}  // namespace content
+
+#endif // CONTENT_BROWSER_MEDIA_SESSION_MEDIA_SESSION_DELEGATE_H_