service worker: Move GetOrCreateRegistration to ServiceWorkerRegistry

This method depends on in memory representation of some service
worker objects (e.g. ServiceWorkerRegistration and
ServiceWorkerVersion). Move it to ServiceWorkerRegistry.

Bug: 1039200
Change-Id: I2c11ff418504289c85ffbeb570b934069899b56c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2004355
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732319}