Define default but still private MediaSession constructor

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()
-{
-}
-
 MediaSession* MediaSession::create()
 {
     return new MediaSession;
 }
 
 void MediaSession::activate()
 {
 }
 
 void MediaSession::deactivate()
 {
 }
 
 } // namespace blink