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This is the first step to removing _lastRegisteredRequestURL from
CRWWebController. See crbug.com/792515 for rationale.

There are two use cases for _lastRegisteredRequestURL:
1. As the URL of the pending navigation
2. Detect fast back-forward navigations in
   |-webViewLoadingStateDidChange| to explicitly trigger
   |-didFinishNavigation:|

NavigationContext::GetUrl() can already support (1).
NavigationContext::GetPageTransition() cannot current distinguish fast
and slow back-forward navigation. Storing WKNavigationType makes this
possible.

Bug: 789993,792515
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4a919a48bd1d7755d006dbebff305558e885c8f2
Reviewed-on: https://chromium-review.googlesource.com/806766
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522235}