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.

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;
   }

testing/web-platform/meta/html/browsers/windows/iframe-cross-origin-print.sub.html.ini

@@ -2,4 +2,4 @@
   bug:
     if fission: https://bugzilla.mozilla.org/show_bug.cgi?id=1695806
   expected:
-    if fission: FAIL
+    if fission: [PASS, FAIL]