Add CacheStorage interface to ServiceWorkerGlobalScope.

This adds a "nativeCaches" object which provides the CacheStorage API
and IDL. Done as a single CL to separate the review of the
WebServiceWorkerFoo classes out.

After we have a working native cache implementation, we'll rename
"nativeCaches" to caches, and drop the polyfills. Until then, you can
play with it by assigning self.caches = self.nativeCaches.

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

Review URL: https://codereview.chromium.org/381633006

git-svn-id: svn://svn.chromium.org/blink/trunk@178811 bbb929c8-8fbe-4397-9dbb-9b2b20218538