* Mask images were the wrong size for some buttons, leading to background color
  bleed in custom themes.
* Mask images were inconsistent about which borders they included.
* Maximize/restore images had red on the right edges for no obvious reason.
* Close button images colored parts of the button border that no other images
  did.

Bug: none
Change-Id: I9a2e80bb07c3d883cfe104696039457828d07d7f
Reviewed-on: https://chromium-review.googlesource.com/1157702Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579962}