Ignore suggested download filename for cross origin links.

The @download attribute can be used to propose a filename for a
download. If the download was initiated by a different, potentially
untrusted origin, then using the proposed filename from the @download
attribute can be a security concern. Therefore, only expose the proposed
filename after verifying that the request isn't a cross origin request.

The request can still be redirected to a different origin. Therefore the
embedder is responsible for ignoring the suggested name on a cross
origin redirect. The companion CL
https://codereview.chromium.org/246893006/ implements this for
//content.

Depends on Chromium CL: https://codereview.chromium.org/200663002/ for
test_runner changes.

BUG=346744

Review URL: https://codereview.chromium.org/197033005

git-svn-id: svn://svn.chromium.org/blink/trunk@172767 bbb929c8-8fbe-4397-9dbb-9b2b20218538