ServiceWorker polyfill Fetch implementation.

This is by no means a good implementation of Fetch, however it is sufficient
to let us start proceeding with the cache API (and add).

R=falken@chromium.org,jkarlin@chromium.org
TBR=jochen@chromium.org
BUG=381179

Review URL: https://codereview.chromium.org/311343003

git-svn-id: svn://svn.chromium.org/blink/trunk@175846 bbb929c8-8fbe-4397-9dbb-9b2b20218538