Introducing the WebServiceWorkerCacheStorage

A new WebServiceWorkerCacheStorage object is introduced for embedders to
implement the CacheStorage API.

While we have been developing ServiceWorker's CacheStorage API, the
underlying API has changed, and that's updated here as well.

See https://github.com/slightlyoff/ServiceWorker/issues/372 for a
discussion of the changes and motivations, and the spec at
https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#cache-storage
is up to date as of writing.

R=falken@chromium.org,abarth@chromium.org,jkarlin@chromium.org,jochen@chromium.org
BUG=392621

Review URL: https://codereview.chromium.org/424643002

git-svn-id: svn://svn.chromium.org/blink/trunk@179676 bbb929c8-8fbe-4397-9dbb-9b2b20218538