Bug 1716537 - Allow updating viewport dimensions on paginated subdocuments. r=mattwoodrow

As those don't have the same incremental reflow issues as root paginated
documents, and we do need this for remote iframes to update their
viewport.

Depends on D119103

Differential Revision: https://phabricator.services.mozilla.com/D119104

layout/generic/nsSubDocumentFrame.cpp

@@ -747,7 +747,7 @@ void nsSubDocumentFrame::Reflow(nsPresContext* aPresContext,
 
   FinishAndStoreOverflow(&aDesiredSize);
 
-  if (!aPresContext->IsPaginated() && !mPostedReflowCallback) {
+  if (!aPresContext->IsRootPaginatedDocument() && !mPostedReflowCallback) {
     PresShell()->PostReflowCallback(this);
     mPostedReflowCallback = true;
   }