Add MediaSession runtime flag along with stub MediaSession object

Source/modules/mediasession/MediaSession.h

Index: Source/modules/mediasession/MediaSession.h
diff --git a/Source/modules/mediasession/MediaSession.h b/Source/modules/mediasession/MediaSession.h
new file mode 100644
index 0000000000000000000000000000000000000000..e7dd0cf5c4f7ac856ce34697f86d8c662213656c
--- /dev/null
+++ b/Source/modules/mediasession/MediaSession.h
@@ -0,0 +1,27 @@
+// 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 MediaSession_h
+#define MediaSession_h
+
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class MediaSession : public ScriptWrappable, public RefCountedWillBeNoBase<MediaSession> {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static RefPtrWillBeRawPtr<MediaSession> create();
+
+    void activate();
+    void deactivate();
+
+private:
+    MediaSession();
+};
+
+}
+
+#endif // MediaSession_h