I remember sgabriel@ mentioned offline that we will show 1px background blur between notifications.

I'm actually afraid that putting a padding between notifications will make the separator too dark as the background is pretty dark. I was considering doing something like Android does which is have a grey background underneath the notifications so that the 1dip padding between them appears grey and we can control the coloring. I'll try to come up with a spec for that. The current color I'd like to try is #DFE0E0

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b53e06c09df102d925273795aa89e83b3b285966

commit b53e06c09df102d925273795aa89e83b3b285966
Author: Tetsui Ohkubo <tetsui@chromium.org>
Date: Wed May 30 10:03:24 2018

Round notifications in UnifiedSystemTray.

This CL rounds notification corners properly and adds separators between
notifications.

The notifications at the top and at the bottom should have rounded
corners. This CL
* Adds MessageView::UpdateCornerRadius(). Some subclasses e.g. ARC++
  notification does not use default background of MessageView, so they
  override this method to implement their own corner rounding.
* Removes sidebar_enabled flag and set border directly from
  UnifiedMessageCenterView. 1px #DFE0E0 solid border is added between
  notifications. (The color is not final but mentioned in the bug.)

Screenshot: http://screen/ZbG9fzvZZ13
UX spec: http://shortn/_LO6ro9tLZK

BUG= 846181 
TEST=manual

Change-Id: Ib36ff38955a9d9351726134b4294c380570c3a90
Reviewed-on: https://chromium-review.googlesource.com/1078408
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562782}
[modify] https://crrev.com/b53e06c09df102d925273795aa89e83b3b285966/ash/message_center/message_center_view.cc
[modify] https://crrev.com/b53e06c09df102d925273795aa89e83b3b285966/ash/system/tray/tray_constants.h
[modify] https://crrev.com/b53e06c09df102d925273795aa89e83b3b285966/ash/system/unified/unified_message_center_view.cc
[modify] https://crrev.com/b53e06c09df102d925273795aa89e83b3b285966/ui/message_center/views/message_view.cc
[modify] https://crrev.com/b53e06c09df102d925273795aa89e83b3b285966/ui/message_center/views/message_view.h