script: Support decomposing ShadowRoot from mozjs HandleValue (#38984)

- Add `ShadowRoot` to `JSValue` to avoid
`WebDriverJSError::UnknownType`, and
`JavaScriptEvaluationError::SerializationError` when execute JS from
embedder.
- Add unit test.
- Move [is_detached](https://w3c.github.io/webdriver/#dfn-is-detached)
to `fn is_detached` to be reused.
- Other random simplification.

Testing: WebDriver conformance tests.

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/script/dom/htmlelement.rs

@@ -617,7 +617,7 @@ impl HTMLElementMethods<crate::DomTypeHolder> for HTMLElement {
         // Note: the element can pass this check without yet being a custom
         // element, as long as there is a registered definition
         // that could upgrade it to one later.
-        let registry = self.owner_document().window().CustomElements();
+        let registry = self.owner_window().CustomElements();
         let definition = registry.lookup_definition(self.as_element().local_name(), None);
 
         // Step 3: If definition is null, then throw an "NotSupportedError" DOMException

components/script/dom/node.rs

@@ -1282,7 +1282,7 @@ impl Node {
     pub(crate) fn summarize(&self, can_gc: CanGc) -> NodeInfo {
         let USVString(base_uri) = self.BaseURI();
         let node_type = self.NodeType();
-        let pipeline = self.owner_document().window().pipeline_id();
+        let pipeline = self.owner_window().pipeline_id();
 
         let maybe_shadow_root = self.downcast::<ShadowRoot>();
         let shadow_root_mode = maybe_shadow_root