[ServiceWorker] CacheStorage tests.

These tests exercise the following methods of CacheStorage:
  * get()
  * has()
  * create()
  * delete()
  * keys()

The editors draft is at:
https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#cache-storage

BUG=374802

Review URL: https://codereview.chromium.org/430993002

git-svn-id: svn://svn.chromium.org/blink/trunk@182633 bbb929c8-8fbe-4397-9dbb-9b2b20218538