Bug 1169881 - Recompute image visibility when display port margins change. r=tn

layout/base/nsLayoutUtils.cpp

@@ -1110,6 +1110,10 @@ nsLayoutUtils::SetDisplayPortMargins(nsIContent* aContent,
     }
   }
 
+  // Display port margins changing means that the set of visible images may
+  // have drastically changed. Schedule an update.
+  aPresShell->ScheduleImageVisibilityUpdate();
+
   return true;
 }