ServiceWorker: Make a way to retrieve a service worker version id (Blink)

This is used to make a key of MemoryCache:
https://codereview.chromium.org/640463003/

Chromium-side: https://codereview.chromium.org/669303003/

BUG=423219
TEST=n/a

Review URL: https://codereview.chromium.org/650033007

git-svn-id: svn://svn.chromium.org/blink/trunk@184561 bbb929c8-8fbe-4397-9dbb-9b2b20218538