auto merge of #1915 : Ms2ger/servo/wrap-return-js, r=jdm

This lets us avoid the sketchy tricks in JS::new and Window::new, where we
kept an unsafe pointer to the native object across the Wrap call that
consumed the owned pointer.

src/components/script/dom/bindings/utils.rs

@@ -424,7 +424,7 @@ pub trait Reflectable {
 pub fn reflect_dom_object<T: Reflectable>
         (obj:     ~T,
          window:  &JS<window::Window>,
-         wrap_fn: extern "Rust" fn(*JSContext, &JS<window::Window>, ~T) -> *JSObject)
+         wrap_fn: extern "Rust" fn(*JSContext, &JS<window::Window>, ~T) -> JS<T>)
          ->       JS<T> {
     JS::new(obj, window, wrap_fn)
 }