This part is mainly to mark the channel as urgent-start if src related
attributes in HTMLImageElement and HTMLInputElement is set and the channel is
open due to user interaction. Unfortunately, we cannot just check the event
state just after creating channel since some loading image tasks will be queue
and execute in stable state. Thus, I store the event state in elements and
pass it to the place where create the channel.
MozReview-Commit-ID: GBdAkPfVzsn
