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In preparation to make the NTP a contained view controller of the BVC, add
helper classes to track activating and deactivating the NTP from a tab helper.

Currently the NTP is displayed as native content within the web content area via
CRWNativeContent.  Rather than go thru ios/web for a native page, instead
create and use a new NTP tab helper and display the NTP directly from within
ios/chrome.

Historically the NTP has been a collection of fake view controllers predating
view controller containment.  While much of the NTP hierarchy has been updated,
this will be the final change required to have a normal NTP presentation with
normal UIViewControllers.

Bug: 826369
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I8517c0231dbfdf42185a7d82ce4ca009e9002bd9
Reviewed-on: https://chromium-review.googlesource.com/c/1280046
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599690}