Add stub for assigning media session to media elements

Source/modules/mediasession/HTMLMediaElementMediaSession.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 HTMLMediaElementMediaSession_h
+#define HTMLMediaElementMediaSession_h
+
+#include "core/html/HTMLMediaElement.h"
+#include "modules/ModulesExport.h"
+#include "modules/mediasession/MediaSession.h"
+#include "platform/Supplementable.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class HTMLMediaElementMediaSession final : public NoBaseWillBeGarbageCollected<HTMLMediaElementMediaSession>, public WillBeHeapSupplement<HTMLMediaElement> {
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(HTMLMediaElementMediaSession);
+public:
+    static MediaSession* session(HTMLMediaElement&);
+    static void setSession(HTMLMediaElement&, MediaSession*);
+
+    static HTMLMediaElementMediaSession& from(HTMLMediaElement&);

[philipj_slow, 2015/07/02 08:53:15]

These two could possibly be private?

+    static const char* supplementName();
+
+    DECLARE_VIRTUAL_TRACE();
+
+private:
+    PersistentWillBeMember<MediaSession> m_session;
+};
+
+} // namespace blink
+
+#endif // HTMLMediaElementMediaSession_h