Auto merge of #224 - dlrobertson:atomic_usize_init, r=jdm
Auto merge of #224 - dlrobertson:atomic_usize_init, r=jdm

unix: ATOMIC_USIZE_INIT is now deprecated

`ATOMIC_USIZE_INIT` is now deprecated. The `AtomicUsize::new` function is
now preferred. Use `AtomicUsize::new(0)` instead of `ATOMIC_USIZE_INIT`.

servo/ipc-channel