Bug fix:
- Fix the width of MessageCenterBuble (web_notification_tray.cc). The previous change (https://crrev.com/c/722303) changed the border thickness, but that forgot to update the width.

Clean-ups (not behavior change):
- Rename margin related constants
  * message_center::kMarginBetweenItems -> kMarginBetweenPopups
  * message_center_style::kMarginBetweenItems -> kMarginBetweenItemsInList

# TBRing for just renaming constant in ash/.

Bug: 726246
Change-Id: I850323cfab1febba0e59ff37c46db386a116a374
TBR: shuhne@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/737690


Commit-Queue: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511531}