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This enables us to remove the NTP as quickly as possible, without
needing to add a single-use WebStateObserver callback.  This only
affects the kBrowserContainerContainsNTP feature.

This CL also removes the use of -GetPendingItem in the NewTabPageTabHelper.

Bug: 826369
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I0895b6109eead0aba3cd0f95bd8c1e97e1c4be28
Reviewed-on: https://chromium-review.googlesource.com/c/1283839
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600270}