Native Welcome Notification for Chrome Now and Synced Notifications

Users receiving notifications from components they did not explicitly
install will receive a welcome notification to explain where these
notifications originate.

Users will see this notification until it is dismissed. This dismissal
propagates to other machines with the help of synced preferences.

BUG=305449

Review URL: https://codereview.chromium.org/26601012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229705 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229705