Once we paint, it's way too late for this check to happen anyway.
Additionally, the spec's steps for retrieving the content document
assume that both the browsing context's active document and the
container's node document are non-null, which evidently isn't always the
case here, as seen by crashes on the SerenityOS 2nd and 3rd birthday
pages (I'm not sure about the details though).
Fixes#12565.
The "completely finish loading" algorithm (from the HTML spec) is
responsible for sending a "load" event to nested browsing context
containers (iframes).
This patch removes the old mechanism for sending "load" events, which we
had mistakenly kept around, causing two events to be sent instead of
one. :^)
This changes allows for nested browser contexts to be embedded in the
serialized JSON of their container element (like `iframe`) and enables
their inspection in the DOM Inspector.
