Add ServiceWorkerCacheStorage implementation of NameToCacheMap

This is in the spec as
https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#name-to-cache-map-internal-interface
, using this logic also greatly simplifies a public interface in the downstream
WebServiceWorkerCache CL.

R=jsbell@chromium.org,jkarlin@chromium.org
BUG=399178

Review URL: https://codereview.chromium.org/593893004

git-svn-id: svn://svn.chromium.org/blink/trunk@182544 bbb929c8-8fbe-4397-9dbb-9b2b20218538