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Under Refresh, as tabs are hovered and the animating hover highlight
is active, several highlighted tabs can be visible. This will change
the painting order of those tabs so that the lower corners of the tabs
are painted such that they aren't likely to look like a "speech bubble"
as pbos@ referred to them.

As a consequence, last_hovered_tab_ could be removed since it's not
needed with the new logic.

Bug: None
Change-Id: I52c470d22966241e47b8450161e4ab9f31966e1b
Reviewed-on: https://chromium-review.googlesource.com/1140954
Commit-Queue: Allen Bauer <kylixrd@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576334}