To avoid conflict with handle mapping macros.

The new spec uses length attribute for some static arrays. Change is_dynamic_array
to account account for that. Eventually we could use the new information to improve
conversion thunks.

The event target may be from a different document to the document associated with the
event handler.

Use generic event dispatcher instead of nsevent for the
DOMContentLoaded event.

Also allow processing before dispatching event.

Only update dom_content_loaded_event start/end time when the event is
trusted.