[Fetch API] Add tests of 304 handling for fetch().

This is the same as what xmlhttprequest/cache-override.html checks for XMLHttpRequest. 
See: https://github.com/slightlyoff/ServiceWorker/issues/412

BUG=None
TEST=http/tests/serviceworker/fetch-cache-override.html

Review URL: https://codereview.chromium.org/580023002

git-svn-id: svn://svn.chromium.org/blink/trunk@182322 bbb929c8-8fbe-4397-9dbb-9b2b20218538