Implements click-to-disable for synced notifications welcome toasts.

Services will clean themselves out of the message center when the button
is clicked.  This requires closing the notification center since changes
are queued while it's open.

BUG=315248

Review URL: https://codereview.chromium.org/145133010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247145 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247145