ServiceWorker, remove old Cache API implementation to make way for the polyfill.

R=falken@chromium.org,jkarlin@chromium.org
BUG=374822

Review URL: https://codereview.chromium.org/311233007

git-svn-id: svn://svn.chromium.org/blink/trunk@175596 bbb929c8-8fbe-4397-9dbb-9b2b20218538