[iOS] Fix WebState callbacks for new Error page workflow

This CL makes sure that the WebState callbacks (DidStartNavigation,
DidFinishNavigation...) are correctly called with the new Error page
workflow.
The callbacks are called once per error, with the URL of the page
requested.

Fixed: 1081484, 991608
Change-Id: I92934ea36c270f6f99fb355011b87083b8592988
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398130


Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805673}