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This CL updates the tab strip's UpdateNewTabButtonBorder() method to
apply the vertical border insets to the tab control buttons rather
than the tab controls container.

This is necessary as the NewTabButton will extend its targetable
area to the top of its bounds when the tab strip is in a condensed
state (i.e. the window is maximized). This is done to ensure the
tab strip controls comply with Fitt's law expectations.

Bug: 1136557
Change-Id: I8b312363bdabbef358ec2689321bb30d7680953b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463994
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815821}