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This CL creates a simple grid view controller.
It also adds an entry in showcase.

Partly to avoid name collision, we name the single grid
as a GridViewController. The parent tab switcher with multiple
panels and other controls is called the TabGridViewController.

Bug: 804496
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib5b06befc35408958b8613a776f2c93742137bc7
Reviewed-on: https://chromium-review.googlesource.com/920641
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537213}