Support actual-source-URL relative downloadURL and updateURL meta.
Support actual-source-URL relative downloadURL and updateURL meta.

Fixes #1658

Martii/greasemonkey,Ventero/greasemonkey,the8472/greasemonkey,HasClass0/greasemonkey,the8472/greasemonkey,HasClass0/greasemonkey,Ventero/greasemonkey,Martii/greasemonkey