Add MediaSession runtime flag along with stub MediaSession object

Source/modules/mediasession/MediaSession.cpp

+// 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.
+
+#include "config.h"
+#include "modules/mediasession/MediaSession.h"
+
+namespace blink {
+
+MediaSession::MediaSession()
+{
+}
+
+RefPtrWillBeRawPtr<MediaSession> MediaSession::create()
+{
+    return adoptRefWillBeNoop(new MediaSession());
+}
+
+void MediaSession::activate()
+{
+}
+
+void MediaSession::deactivate()
+{
+}
+
+}