Backed out changeset e79968b5fd84 (bug 1276107) for often failing test_group_wheelevents.html with e10s. r=backout

layout/generic/nsGfxScrollFrame.cpp

@@ -2845,13 +2845,7 @@ MaxZIndexInListOfItemsContainedInFrame(nsDisplayList* aList, nsIFrame* aFrame)
 {
   int32_t maxZIndex = -1;
   for (nsDisplayItem* item = aList->GetBottom(); item; item = item->GetAbove()) {
-    nsIFrame* itemFrame = item->Frame();
-    // Perspective items return the scroll frame as their Frame(), so consider
-    // their TransformFrame() instead.
-    if (item->GetType() == nsDisplayItem::TYPE_PERSPECTIVE) {
-      itemFrame = static_cast<nsDisplayPerspective*>(item)->TransformFrame();
-    }
-    if (nsLayoutUtils::IsProperAncestorFrame(aFrame, itemFrame)) {
+    if (nsLayoutUtils::IsProperAncestorFrame(aFrame, item->Frame())) {
       maxZIndex = std::max(maxZIndex, item->ZIndex());
     }
   }