Bug 1356103 - Part 6: Make gfxUserFontSet refcounting thread-safe. r=bholley

As with a few other gfx* font-related classes, during font metrics
calculations we end up taking strong references to gfxUserFontSet,
and it would be difficult to restructure the code to not do this.

MozReview-Commit-ID: L1GbZnf4825

--HG--
extra : rebase_source : 3bc2deb24e282f4a76f0a270d28771016052f9ec

gfx/thebes/gfxUserFontSet.h

@@ -175,7 +175,7 @@ class gfxUserFontSet {
 
 public:
 
-    NS_INLINE_DECL_REFCOUNTING(gfxUserFontSet)
+    NS_INLINE_DECL_THREADSAFE_REFCOUNTING(gfxUserFontSet)
 
     gfxUserFontSet();