The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7e2b5c14fa70447b692fab4f7a9660729cf55b38

commit 7e2b5c14fa70447b692fab4f7a9660729cf55b38
Author: estade <estade@chromium.org>
Date: Fri Jan 20 19:33:48 2017

Fix gtk theme regression introduced in 477aca494328f

Turns out we do need to explicitly clear the border images. This bug is
visible when you hover a toolbar button.

BUG= 682940 

Review-Url: https://codereview.chromium.org/2639423005
Cr-Commit-Position: refs/heads/master@{#445130}

[modify] https://crrev.com/7e2b5c14fa70447b692fab4f7a9660729cf55b38/chrome/browser/ui/libgtkui/gtk_ui.cc

A friendly reminder that M57 Beta launch is coming soon on February 2nd! Your bug is labelled as Beta ReleaseBlock, pls make sure to land the fix and get it merged into the release branch (2987) ASAP so it gets enough baking time in Dev (before Beta promotion). Thank you!

Your change meets the bar and is auto-approved for M57. Please go ahead and merge the CL to branch 2987 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), ketakid@(cros), govind@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Please merge your change to M57 branch 2987 ASAP (if possible before 5:00 PM PT today).So we can pick it for Dev release tomorrow (02/24).

https://codereview.chromium.org/2645423002

bugdroid is slacking off

Per comment #6, this is already merged to M57.