GitLab

Validate that the |changed_button_flags_| field is allowed only the
set of flags that makes sense.

BUG=779372, 824629

Change-Id: I5e4d8b8f07fbf6d881d8510f45414814d074278d
Reviewed-on: https://chromium-review.googlesource.com/974078
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545120}