don't leave downloadUrl null when both video and audio urls are available.
Split 'common/' into commit 'cbd8c76aa4dc51f4d40ef20b696bd0096d403d2a'

git-subtree-dir: common
git-subtree-mainline: 6b336f82b682cdd1dccaa6efc4b9d4f882552d4a
git-subtree-split: cbd8c76aa4dc51f4d40ef20b696bd0096d403d2a