Simplify WebServiceWorker{Registration}Proxy interfaces.

Turn WebServiceWorkerRegistrationProxy and WebServiceWorkerProxy into
pure abstract interfaces, letting the embedder call the "proxy"
interfaces on the ServiceWorkerRegistration and ServiceWorker Blink
objects, respectively. That is, no longer keep private (and bare)
pointers on these platform proxy objects; unnecessary and runs up
against overall platform layering constraints.

Like before, the ownership of these proxy objects resides with Blink.

R=nhiroki,haraken,mkwst
BUG=410257

Review URL: https://codereview.chromium.org/682003003

git-svn-id: svn://svn.chromium.org/blink/trunk@184587 bbb929c8-8fbe-4397-9dbb-9b2b20218538