Bug 1568772. Fix "Multiple glass backgrounds found?" warning. r=mattwoodrow

If we re-use the old item we hit the warning because we try to set the very same item as the glass item, but that is fine.

Differential Revision: https://phabricator.services.mozilla.com/D39308

--HG--
extra : moz-landing-system : lando

layout/painting/RetainedDisplayListBuilder.cpp

@@ -474,7 +474,9 @@ class MergeState {
         }  // aNewItem can't be the glass item on the builder yet.
 
         if (destItem->IsGlassItem()) {
-          mBuilder->Builder()->SetGlassDisplayItem(destItem);
+          if (destItem != oldItem || destItem != mBuilder->Builder()->GetGlassDisplayItem()) {
+            mBuilder->Builder()->SetGlassDisplayItem(destItem);
+          }
         }
 
         MergeChildLists(aNewItem, oldItem, destItem);