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 HTML Origin spec has two similar but slightly different concepts of
origin equality: "same origin" and "same origin-domain". Let's be
explicit with the naming here :^)
Also add spec comments.
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. :^)
Any browsing context that doesn't have a parent browsing context is now
considered a top-level browsing context. This matches the HTML spec.
This means we no longer keep a pointer to the top-level context, since
we can simply walk the parent chain until we find the topmost ancestor.
