Bug 1688004 - Build top layer items for XUL documents that don't have scrollframes. r=miko

This will allow the browser chrome to use `<dialog>` etc, see
bug 1685313.

Tweak the check to support scrolling="false" on reftests, so that we can
test this.

Differential Revision: https://phabricator.services.mozilla.com/D102623

layout/base/nsCSSFrameConstructor.cpp

@@ -2565,7 +2565,7 @@ void nsCSSFrameConstructor::SetUpDocElementContainingBlock(
     isScrollable = presContext->HasPaginatedScrolling();
   } else if (isXUL) {
     isScrollable = false;
-  } else if (nsContentUtils::IsInChromeDocshell(aDocElement->OwnerDoc()) &&
+  } else if (aDocElement->OwnerDoc()->AllowXULXBL() &&
              aDocElement->AsElement()->AttrValueIs(
                  kNameSpaceID_None, nsGkAtoms::scrolling, nsGkAtoms::_false,
                  eCaseMatters)) {