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The pre-refresh use of "endcap" isn't quite as clear in refresh, where tabs
appear to start suddenly at a point that turns out to be halfway through the
"endcap".  Instead, restrict usage of GetTabEndcapWidth() (now renamed for
parallelism with GetTabEndcapWidthForPainting()) to pre-refresh.  In refresh,
use GetCornerRadius(), whose relationship to the tab's appearance is more clear.

This adds a new GetContentsInsets() static method as well, which allows
decoupling the concept of the endcap width/corner radius from "how far into the
tab bounds does content start", which is what a number of callers really care
about.  Removing dependencies on GetMinimumInactiveTabWidth() will be useful for
allowing narrower tabs.

Bug: none
Change-Id: Ic749c4b3f1b2c3611db0fa4966d62513367aec51
Reviewed-on: https://chromium-review.googlesource.com/1121818Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572024}