Push API: pass along service worker provider during registration.

To be submitted after: https://codereview.chromium.org/330893003/
BUG=350377

Review URL: https://codereview.chromium.org/334933002

git-svn-id: svn://svn.chromium.org/blink/trunk@177875 bbb929c8-8fbe-4397-9dbb-9b2b20218538