Blink side of exposing the service worker registration associated with geofencing API calls.
This still leaves the old codepaths for the geofencing API in place, those will be cleaned up in a followup CL.

BUG=383125

Review URL: https://codereview.chromium.org/623813002

git-svn-id: svn://svn.chromium.org/blink/trunk@183931 bbb929c8-8fbe-4397-9dbb-9b2b20218538