[iOS] Remove PageNotAvailableController

This CL removes the PageNotAvailableController.
This controller was only instatiated in BVC, when no other
NativeController were available for a specific URL.
However, all the possible cases of displaying a NativeContent are
guarded with a check on the provider to know if it can display a
native content for the URL.

So if there was a case where it was possible to display this controller,
the DCHECK would have been hit.

Bug: 725239, 944008
Change-Id: Iace04dea9f85bed31cec626e0098f8cf1ef9bc71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1533905
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643374}
7 files changed