Grab SecurityOrigin directly.  Avoid URL->Origin conversions.

This CL tweaks MediaControlDownloadButtonElement::DefaultEventHandler to
directly grab the SecurityOrigin of a Document as follows:

     request.SetRequestorOrigin(GetDocument().GetSecurityOrigin());

This avoids incorrectly attempting to convert a Document's URL into an
Origin (which would give incorrect results for about:blank or
about:srcdoc documents, as well as for sandboxed iframes).

Fixed: 1065097
Change-Id: I94e0e375e48b43d2d57da7f78f473cb2cdcfad08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122811
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754531}