GitLab

The message center already does this for notifications with a higher
priority than the notification they replace, but this makes it explicit
for Web Notifications (which cannot indicate priority).

TBR=dewittj (reviewed in cl 301713002)
BUG=366102

Review URL: https://codereview.chromium.org/439073002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287682 0039d316-1c4b-4281-b951-d872f2087c98