Rename Regiser/Unregister and revise the function arguments

1. Register()   -> AddObserver()
2. Unregister() -> RemoveObserver()
3. Use two observer lists and observer pointers to handle always/non-always
   trigger case
force turn on vsync at host.
we have problem for RD in apz case. with apz, the RD is still in-active. if we only turn on RD module, we will stop the vsync event source. so, we always turn on for workaround.
Bug 987529 - part 2: Integrate refresh driver into vsync dispatcher

Integrate refresh drivers on Chrome/Content process into the
vsync dispatcher.