Rename WebServiceWorkerRegistry to WebServiceWorkerProvider

After working on the chromium side, we changed the model a bit.
- Now the WebFrameClient method is more of a factory, and the WebServiceWorkerProvider (formerly the Registry) is owned per-page
- There's now a per-page client so that errors that originate from the browser side can bubble back out to the renderer.

The client interface hasn't been flushed out, but there's a stub didFailToStart() call, that
is the only known error that can propagate from the browser

BUG=285976

Review URL: https://codereview.chromium.org/26078002

git-svn-id: svn://svn.chromium.org/blink/trunk@159543 bbb929c8-8fbe-4397-9dbb-9b2b20218538