Avoid unnecessary repaints on tabstrip buttons

We should avoid `View::SetBorder` calls if unnecessary since it triggers
repainting. This CL changes `TabStripRegionView::UpdateButtonBorders` so
that it won't trigger SetBorder() unless the border insets needs to be
updated.

Bug: b:433590858
Change-Id: If7f9fa80d964ce5e6d501dcb82bc0cbf8fa51b18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6780180
Commit-Queue: Jun Ishiguro <junis@google.com>
Reviewed-by: David Pennington <dpenning@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1491113}