bug 950590 - part 1 - use nsRefPtr instead of manual addref/release calls for gfxFontGroup's reference to the user font set. r=roc a=lsblakk
bug 950590 - part 2 - make SetUserFontSet support updating a gfxFontGroup's user font set on the fly, and use this in canvas rendering context. r=roc a=lsblakk