Clear ServiceWorker CacheStorage promises immediately after use.

By clearing the resolver immediately, double resolution errors should
be caught a lot more quickly.

R=jsbell@chromium.org,dominicc@chromium.org,falken@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/461343005

git-svn-id: svn://svn.chromium.org/blink/trunk@180740 bbb929c8-8fbe-4397-9dbb-9b2b20218538