Bug 1728699. Use IsRootContentDocumentCrossProcess in one place in ScrollFrameHelper::BuildDisplayList. r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D124282

layout/generic/nsGfxScrollFrame.cpp

@@ -3708,8 +3708,7 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
   // of the viewport, so most layer implementations would create a layer buffer
   // that's much larger than necessary. Creating independent layers for each
   // scrollbar works around the problem.
-  bool createLayersForScrollbars =
+  bool createLayersForScrollbars = isRootContent;
-      mIsRoot && mOuter->PresContext()->IsRootContentDocument();
 
   nsIScrollableFrame* sf = do_QueryFrame(mOuter);
   MOZ_ASSERT(sf);