Initial ServiceWorker Cache API polyfill.

A basic Cache API. No serialization of any kind, no testing, and no
add() method, as it depends on Fetch().

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

Review URL: https://codereview.chromium.org/314133002

git-svn-id: svn://svn.chromium.org/blink/trunk@175749 bbb929c8-8fbe-4397-9dbb-9b2b20218538