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.
