[ios] Add tab grid to view controller and showcase

Bug: 804496, 804497
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I05b6627c4eb9ab6ae33e337631b7f1e70466730c
Reviewed-on: https://chromium-review.googlesource.com/924650
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537485}
5 files changed