Bug 1576714 - Don't update scrollbar visibility for non-top-level remote browsers. r=nika

This flag is only meant for window.open() stuff, so not relevant to iframes at
all.

This preserves the current fission behavior (which is quite broken) of always
showing scrollbars.

The way to control scrollbars for iframes (the scrolling attribute) is not
handled at all for Fission, I filed a bug and left a few FIXMEs.

Differential Revision: https://phabricator.services.mozilla.com/D49292

dom/html/nsGenericHTMLFrameElement.cpp

@@ -294,9 +294,9 @@ nsresult nsGenericHTMLFrameElement::AfterSetAttr(
   if (aNameSpaceID == kNameSpaceID_None) {
     if (aName == nsGkAtoms::scrolling) {
       if (mFrameLoader) {
+        // FIXME(bug 1588791): This should work for fission iframes.
         nsIDocShell* docshell = mFrameLoader->GetExistingDocShell();
-        nsCOMPtr<nsIScrollable> scrollable = do_QueryInterface(docshell);
-        if (scrollable) {
+        if (nsCOMPtr<nsIScrollable> scrollable = do_QueryInterface(docshell)) {
           int32_t cur;
           scrollable->GetDefaultScrollbarPreferences(
               nsIScrollable::ScrollOrientation_X, &cur);