GitLab

The new constants are necessary because GTK differentiates between
dropdowns (which it calls comboboxes) and menus.  On the Ambiance
theme (the default on Ubuntu), dropdowns are light but menus are dark,
so this change is necessary for consistency with GTK apps.

BUG=914974
R=pkasting

Change-Id: I4805526eee93322c4e7d57b879e0528b0998c933
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2069021
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744070}