[ios] Validate PendingItem when updating NavigationItem in NTP helper.

GetPendingItem() may not return the NavigationItem associated with a
NavigationContext in some rare circumstances.  Instead, validate that the item
is indeed an NTP about://newtab URL, and update the page again on
-DidFinishNavigation.

Bug: 941245
Change-Id: I4e10091259dc4438b2e33e3c50e6b60a8e2f1fd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520699
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#640420}
3 files changed