Fix issue of localDescription and remoteDescription getter.

Source/modules/mediastream/RTCSessionDescription.idl

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 20 matching lines @@
 // http://www.w3.org/TR/webrtc/#idl-def-RTCSessionDescription
 
 enum RTCSdpType {
     "offer",
     "pranswer",
     "answer"
 };
 
 [
     GarbageCollected,
+    DependentLifetime,
     Constructor(optional RTCSessionDescriptionInit descriptionInitDict)
 ] interface RTCSessionDescription {
     attribute RTCSdpType? type;
     attribute DOMString? sdp;
     serializer = {attribute};
 };