Change background painter to use enclosed rect

The background painter used to use enclosing rect instead of enclosed
rect. This causes an issue when a border is applied, because it is
possible for edges (namely right and bottom edge) to be draw outside of
the canvas. This results in a 1px clipping along those edges.

This is mainly noticeable on ChromeOS devices because some have a scale
factor by default. This is also made more noticeable because of the
round buttons in the 2023 refresh.

Fixed: 1487188
Change-Id: I4122b89d75c2e8cb9cfa5db56e5705c14518a153
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4944679
Commit-Queue: Fred Shih <ffred@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: David Yeung <dayeung@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1211181}