Walk up frame tree for srcdoc referrer policies

When deciding the referrer policy for a srcdoc document, walk up the
frame tree until we find a non-srcdoc document OR a srcdoc document with
its own policy set via a meta element.

This implements the algorithm defined in
https://html.spec.whatwg.org/multipage/browsers.html#set-up-a-browsing-context-environment-settings-object. However,
the spec'ed algorithm has to be adjusted per
https://github.com/whatwg/html/pull/1559#issuecomment-251767893 to
account for meta elements in srcdoc documents (which this CL
implements).

BUG=653034,637007

Review-Url: https://codereview.chromium.org/2400443004
Cr-Commit-Position: refs/heads/master@{#423538}
7 files changed