[ios] Immediately hide the NTP when navigating away from it.

Remove the NTP and show the web view more quickly.  WKWebView appears
to run more slowly in certain circumstances when it is obscured by
another view.

Change-Id: I769d9e5602a0b19a0032d5bca2fc6c6d85f3d365
Reviewed-on: https://chromium-review.googlesource.com/c/1333873
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607705}
3 files changed