DEPS.xwalk: Use the git protocol directly instead of ssh.

Now that the crosswalk repositories are public, we can use direct git:// URLs
to clone them instead of the previous ones that used SSH.

The overhead of using SSH may be negligible in terms of speed, but doing so 
allows truly anonymous checkouts: people may not have a GitHub account (or 
have their SSH keys in the machine they're using at the moment).