[ios] Fix voiceover for NTP

Voiceover reads nothing for NTP tabs in the tab grid.
It should read 'new tab'. Recently, the titles for NTP
tabs were removed to improve the look of the tab grid.
However, this meant that voiceover had nothing to read.

This CL brings back titles for NTP, but instead hides them.
This allows voiceover to read the title, even though it
is not visible.

Bug: 868401
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I37fe3782e6ee68822e59ff1de7ef18e619cece42
Reviewed-on: https://chromium-review.googlesource.com/1152239
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579266}
6 files changed