Add PrerenderStatusEvent on Prerenders
Changes the events a bit as they are passed down, and unifies the PrerenderingSupport and PrerenderingDispatcher interfaces. Read this together with the WebKit issue https://bugs.webkit.org/show_bug.cgi?id=96474
Also, a few other code cleanups/assertion cleanups I came across while coding this.
This change will need to be staged carefully (the change to the WebPrerenderingSupport interface is going to require around four gardened checkins).