Bug 895009.  Don't pass in NonNull/OwningNonNull arguments to WebIDL binding consumers.  r=peterv
Bug 895974.  Implement ParentNode on document fragments and documents and move previousElementSibling and nextElementSibling to ChildNode.  r=smaug
Bug 895495.  Add a faster way to create JS-implemented WebIDL objects from chrome.  r=mccr8

This adds a Interface._create(targetWindow, chromeObj) static method
that lets a JS-implemented WebIDL object implementing Interface be
created in the window targetWindow using chromeObj as its JS
implementation.