NOT FOR LANDING Introduce and use WebMediaSession

public/platform/modules/mediasession/WebMediaSession.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 WebMediaSession_h
+#define WebMediaSession_h
+
+#include "public/platform/WebCallbacks.h"
+#include "public/platform/modules/mediasession/WebMediaSessionError.h"
+
+namespace blink {
+
+using WebMediaSessionCommandCallback = WebCallbacks<void, const WebMediaSessionError&>;
+
+class WebMediaSession {
+public:
+    virtual ~WebMediaSession() {}
+
+    virtual void activate(WebMediaSessionCommandCallback*) = 0;
+    virtual void deactivate(WebMediaSessionCommandCallback*) = 0;
+};
+
+} // namespace blink
+
+#endif // WebMediaSession_h