GitLab

Adds simple opener-based grouping to the experimental tab controller.

Clicks are not handled for activation. Tabs are rendered overlapped
properly.

Bug: 778461
Change-Id: Ic93cb109875d815018a6197e35b10b358959ac02
Reviewed-on: https://chromium-review.googlesource.com/777735Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517913}