GitLab

- scrollbar fades in and out
- scrollbar now only appears when scrolling
- scrollbar disappears after a short while if the list was not scrolled

Bug: 859228
Change-Id: I5478f387bd733ded02c476d2de05881df6b8d9ea
Reviewed-on: https://chromium-review.googlesource.com/1185800Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585539}