Spec update in https://github.com/whatwg/html/pull/1871

Fixes in https://codereview.chromium.org/2400443004

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/618cf68c2fc06d58857f4bd44a26737fbe9a2494

commit 618cf68c2fc06d58857f4bd44a26737fbe9a2494
Author: estark <estark@chromium.org>
Date: Thu Oct 06 15:23:19 2016

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}

[add] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-srcdoc-dynamic-policy.html
[add] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-srcdoc.html
[modify] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/LayoutTests/http/tests/security/resources/echo-referrer-header.php
[add] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/LayoutTests/http/tests/security/resources/referrer-policy-srcdoc.php
[modify] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/Source/core/dom/Document.h
[modify] https://crrev.com/618cf68c2fc06d58857f4bd44a26737fbe9a2494/third_party/WebKit/Source/core/dom/ExecutionContext.h

[Automated comment] removing mislabelled merge-merged-2840