service worker: Move service_worker.mojom to service_worker_provider.mojom.

The mojom only has the ServiceWorkerDispatcherHost interface now, which
is about service worker providers.

Preparation for moving service_worker_event_dispatcher.mojom to
service_worker.mojom. This might make the git history more sensible.

Bug: 629701
Change-Id: I89693e40ad0d4c259233fb2b805519f432b483ec
Reviewed-on: https://chromium-review.googlesource.com/1142860
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576391}
5 files changed