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This cl revises the tab spacing in vertical mode to match the spacing in
the wallpaper picker.

Images before and after changes are uploaded in the bug.

Bug: 889559
Test: manual.
Change-Id: I33a73809a9c674b009c5fdf1f9367645d395af87
Reviewed-on: https://chromium-review.googlesource.com/1249818Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594896}