Bug 1722991, don't try to load iframes in the background when Fission is enabled r=sefeng

The feature isn't Fission compatible at the moment and it is disabled by default anyhow, so
better try to not use it with Fission so that some crashes can be avoided.

Differential Revision: https://phabricator.services.mozilla.com/D121714

dom/base/DocGroup.cpp

@@ -359,7 +359,8 @@ void DocGroup::SignalSlotChange(HTMLSlotElement& aSlot) {
 }
 
 bool DocGroup::TryToLoadIframesInBackground() {
-  return StaticPrefs::dom_separate_event_queue_for_post_message_enabled() &&
+  return !FissionAutostart() &&
+         StaticPrefs::dom_separate_event_queue_for_post_message_enabled() &&
          StaticPrefs::dom_cross_origin_iframes_loaded_in_background();
 }