Supports per-profile desktop in MessageCenter.

This CL expands NotificationBlocker API and introduces MultiUserNotificationBlockerChromeOS
which filters notifications based on the active desktop.

BUG=314810
R=dewittj@chromium.org, skuhne@chromium.org
TEST=TBD

Review URL: https://codereview.chromium.org/84173003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239086 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239086