Make window.frameElement use the creation context of the parent frame

Previously, accessing window.frameElement inside an <iframe> would cause
the iframe wrapper to use the prototype from its own context, rather
than the context of the frame it's in. This patch fixes that and is
consistent with Firefox and IE (haven't tested on Safari).

This patch doesn't change the cross-origin policy check, which is
covered by an existing test:
http/tests/security/cross-frame-access-frameelement.html

BUG=321032

Review URL: https://codereview.chromium.org/82653003

git-svn-id: svn://svn.chromium.org/blink/trunk@162607 bbb929c8-8fbe-4397-9dbb-9b2b20218538