Add MediaSession runtime flag along with stub MediaSession object

Source/modules/mediasession/MediaSession.cpp

Index: Source/modules/mediasession/MediaSession.cpp
diff --git a/Source/modules/mediasession/MediaSession.cpp b/Source/modules/mediasession/MediaSession.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..644651b5371044e4ba88e6f050e2677b7c1ee60d
--- /dev/null
+++ b/Source/modules/mediasession/MediaSession.cpp
@@ -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.
+
+#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()
+{
+}
+
+}