Add a flag to hide scroll bar in notification list

ARC++ notifications are shown above scroll bar, so we temporarily make
scroll bar invisible as a workaround.

When the limitation is removed from ARC++, the flag will be removed and
the scroll bar will be restored.

TEST=manual
BUG=858940

Change-Id: I6931c6406c9feb2bb4159d737205187bdc88b86b
Reviewed-on: https://chromium-review.googlesource.com/1140014
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576755}
7 files changed