Add blink-side binding code for navigator.serviceWorker.current

Test code is basically copied from falken@'s:
https://github.com/mattto/ServiceWorker/tree/master/examples/layout-tests
(minus one obsolete FIXME, plus another FIXME)

BUG=366538
TEST=http/tests/serviceworker/current-on-load.html
TEST=http/tests/serviceworker/current-on-reload.html

Review URL: https://codereview.chromium.org/265943003

git-svn-id: svn://svn.chromium.org/blink/trunk@173716 bbb929c8-8fbe-4397-9dbb-9b2b20218538