Introducing the WebServiceWorkerCache.

This CL provides the new WebServiceWorkerCache object, which connects the
ServiceWorker Cache between blink and content. There's two notable exceptions
in the implementation; the various Cache.add() and Cache.addAll() methods are
not implemented (they should do fetches in blink and provide the responses to
this API through the batch call), and Response bodies via blobs is not implemented.

R=dominicc@chromium.org,falken@chromium.org,jkarlin@chromium.org
BUG=399178

Review URL: https://codereview.chromium.org/433793002

git-svn-id: svn://svn.chromium.org/blink/trunk@182604 bbb929c8-8fbe-4397-9dbb-9b2b20218538