Add blink interface (and code) for setting navigator.serviceWorker.current

BUG=361344
TEST=to be added when we have chromium code + binding code

Review URL: https://codereview.chromium.org/239973002

git-svn-id: svn://svn.chromium.org/blink/trunk@171753 bbb929c8-8fbe-4397-9dbb-9b2b20218538