Add ServiceWorkerRegistration.id and supporting algorithm changes. (F…

…ixes #1512)

This commit adds support for a registration `id` value.  This will be used to uniquely identify registrations
instead of `scope`.  This will make it easier for a developer to migrate a registration from one `scope` to
another.

The commit consists of the following changes:

* Add a DOMString `id` and associated getter to the registration.
* Add an `origin` to the internal registration type.
* Move the `scope` internal representation to `ServiceWorker` and add a new getter.
* Make the `ServiceWorkerRegistration.scope` return the oldest associated worker's `scope`.
* Migrate the registration map to be keyed by the tuple `(origin,id)`.
* Migrate the job queue map to be keyed by the tuple `(origin,id)`.
* Adjust job equality checks to treat register jobs to account for new id and scope semantics.
* Support changing the scope during register operations.
* Reject register operations that change the scope to a value that is already in use by another registration.
* Properly un-control clients whose URL no longer matches a new active worker's scope.