Pass an argument to spawn_named_with_send_on_failure to support spawning native tasks.
Use spawn_named_with_send_on_failure for the RenderTask (fixes #2744).
Merge pull request #3000 from Ms2ger/rendertask

Use spawn_named_with_send_on_failure for the RenderTask; r=Manishearth
activate wpt-tests on linux travis file