Bug 829248.  Implement basic support for [TreatUndefinedAs=Missing] in WebIDL.  r=khuey

This does not add support for [TreatUndefinedAs=Missing] in overloaded
methods, since it's not clear what that should really look like.

This also does not require that [TreatUndefinedAs=Missing] on an
argument force it on all later arguments, since it's not clear that we
want that long-term.
Bug 861938.  Make the CanvasWindingRule arguments be treated as missing if undefined is passed in.  r=smaug
Bug 683159.  Remove now-irrelevant assertion annotation.  r=ms2ger