Bug 687441 - Make iframes not cause a FOUC on parent document. r=bzbarsky

This makes subframes not start layout on the parent document when flushing,
which fixes the FOUC issue.

It does mean that some resource loads triggered by viewport-dependent media
queries might not block onload of the frame, but other style-triggered loads,
fonts, and such would.

This makes same-origin frames act the same way as cross-origin iframes in this
regard if there are stylesheet loads present.

I think the trade-off is worth it here, and this is green on try.

Differential Revision: https://phabricator.services.mozilla.com/D55860

--HG--
extra : moz-landing-system : lando

dom/base/Document.cpp

@@ -9793,7 +9793,7 @@ void Document::FlushPendingNotifications(mozilla::ChangesToFlush aFlush) {
   // layout flush on our parent, since we need our container to be the
   // correct size to determine the correct style.
   if (StyleOrLayoutObservablyDependsOnParentDocumentLayout() &&
-      IsSafeToFlush()) {
+      mParentDocument->MayStartLayout() && IsSafeToFlush()) {
     mozilla::ChangesToFlush parentFlush = aFlush;
     if (flushType >= FlushType::Style) {
       parentFlush.mFlushType = std::max(FlushType::Layout, flushType);