Post PlzNavigate the provisional load concept no longer makes sense. Ideally, we'd want to remove the concept entirely.

1) We should rename {Did|Will}CommitProvisionalLoad methods to something clearer: DidCommitNavigation and ReadyToCommitNavigation for example.
2) We should deprecate DidStartprovisionalLoad and DidFailProvisionalLoad as they are error-prone post PlzNavigate and follow up with teams still using them about their needs. We should see whether they can use existing navigation observers (e.g. in the browser process) or need to be specifically warned when a renderer requests a navigation from the browser process.
3) The end goal is to remove the provisional DoCumentLoader, which would simplify Blink's loading architecture.