Bug 585258. Part 1: Don't make visible region arbitrarily complex in RecomputeVisibility. r=dbaron,a=blocking

layout/base/nsDisplayList.cpp

@@ -703,7 +703,7 @@ PRBool nsDisplayItem::RecomputeVisibility(nsDisplayListBuilder* aBuilder,
 
   PRBool forceTransparentBackground;
   nsRegion opaque = TreatAsOpaque(this, aBuilder, &forceTransparentBackground);
-  aVisibleRegion->Sub(*aVisibleRegion, opaque);
+  aBuilder->SubtractFromVisibleRegion(aVisibleRegion, opaque);
   return PR_TRUE;
 }