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/webdriver_server/lib.rs

@@ -260,6 +260,7 @@ impl Serialize for SendableJSValue {
             JSValue::Number(x) => serializer.serialize_f64(x),
             JSValue::String(ref x) => serializer.serialize_str(x),
             JSValue::Element(ref x) => WebElement(x.clone()).serialize(serializer),
+            JSValue::ShadowRoot(ref x) => ShadowRoot(x.clone()).serialize(serializer),
             JSValue::Frame(ref x) => WebFrame(x.clone()).serialize(serializer),
             JSValue::Window(ref x) => WebWindow(x.clone()).serialize(serializer),
             JSValue::Array(ref x) => x
@@ -2066,6 +2067,10 @@ impl Handler {
                 ErrorStatus::StaleElementReference,
                 "Stale element",
             )),
+            Err(WebDriverJSError::DetachedShadowRoot) => Err(WebDriverError::new(
+                ErrorStatus::DetachedShadowRoot,
+                "Detached shadow root",
+            )),
             Err(WebDriverJSError::Timeout) => {
                 Err(WebDriverError::new(ErrorStatus::ScriptTimeout, ""))
             },