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Pinned tabs are presented as a grid, with a maximum of 4 tabs stacked
vertically. This CL adds placeholder "ghost" tabs to fill in each
column where there are not enough actually pinned tabs to fill the 4
spaces.

This CL aims to create the placeholders using only HTML and CSS, with
minimum JS to avoid keeping track of a dynamic number of placeholders
and constantly re-creating and removing placeholders.

https://i.imgur.com/0NK8ikk.png

Bug: 989131
Change-Id: I511a7f82fbf4480b53a437dae4b6d9445249514a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772227Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690954}