Re-set referrer on issuing a validation request for a cached resource.

If a cached resource needs to be revalidated, then supply the referrer
of the document making the request.

R=japhet@chromium.org
BUG=344335

Review URL: https://codereview.chromium.org/201973002

git-svn-id: svn://svn.chromium.org/blink/trunk@169392 bbb929c8-8fbe-4397-9dbb-9b2b20218538