GitLab

Removed hard-coded color references for the overlay scrollbar and
added the relevant color ids. Added appropriate selectors for gtk
native theme.

Bug: 1003612
Change-Id: I5c7a8354a151249e2fc5e2a52244c441a15979fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1950861Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722516}