Add a command-line argument to use OSMesa

On Chromoting, the system GL implementation doesn't work properly. In order to
run under Chromoting, we need a way to run with OSMesa instead of the system
GL. This CL adds an argument to the native viewport service to use OSMesa.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/678213002