Make get() and set() on MutNullableHeap use the correct types.

get() must always return a rooted value, because we have no way of
ensuring the value won't be invalidated. set() takes an &T because it's
convenient; there isn't any need to expose JS<T>.

components/script/dom/webglrenderingcontext.rs

@@ -360,7 +360,7 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
 
         if let Some(texture) = texture {
             match texture.bind(target) {
-                Ok(_) => slot.set(Some(JS::from_ref(texture))),
+                Ok(_) => slot.set(Some(texture)),
                 Err(err) => return self.webgl_error(err),
             }
         } else {