Use document referrer policy when preloading

      Previously, preload requests used the referrer policy from meta tags
encountered during scanning, but not from headers delivered with the
page. This CL uses the document's current referrer policy when the
preload scan starts.

BUG=605451

Review-Url: https://codereview.chromium.org/1913983002
Cr-Commit-Position: refs/heads/master@{#390264}