fix(client): Allow Fx18 to use the iframe flow.

Fx18 had two problems:
1) anchor.origin is not available and must be built from anchor.protocol and anchor.host.
2) anchor.protocol and anchor.host are only updated if the anchor's href is set via `anchor.setAttribute(href, ...`.

fixes #1969
Merge pull request #1970 from mozilla/issue-1969-iframe-fx18

fix(client): Allow Fx18 to use the iframe flow.