Use window.location instead of iframe to trigger download.

Bugs:

 * https://bugzilla.mozilla.org/show_bug.cgi?id=926701
 * https://bugzilla.mozilla.org/show_bug.cgi?id=924824

This is needed because an iframe is blocked by Chrome if the page is on HTTPS and the iframe contents redirect to an HTTP URI. See
Merge pull request #1319 from gauthierm/fix-blocked-download-iframe-chrome

Bug 926701, 924824: Use window.location instead of iframe to trigger download.