[ios] Beginnings of tab grid mediator

Bug: 804496
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ifed54179a9f9ffb5e61f270664af35a3f86c9deb
Reviewed-on: https://chromium-review.googlesource.com/924532
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537509}
6 files changed