[webdriver] Implement get shadow root (#37280)

Implement Get Element Shadow Root
https://www.w3.org/TR/webdriver2/#dfn-get-element-shadow-root

cc: @xiaochengh, @yezhizhen, @PotatoCP 

Testing:
`\tests\wpt\tests\webdriver\tests\classic\get_element_shadow_root\get.py`
is blocked by `no_browsing_context` and `closed_window`
pass for other sub-tests.

Signed-off-by: batu_hoang <longvatrong111@gmail.com>

components/script/script_thread.rs

@@ -2280,6 +2280,14 @@ impl ScriptThread {
                     can_gc,
                 )
             },
+            WebDriverScriptCommand::GetElementShadowRoot(element_id, reply) => {
+                webdriver_handlers::handle_get_element_shadow_root(
+                    &documents,
+                    pipeline_id,
+                    element_id,
+                    reply,
+                )
+            },
             WebDriverScriptCommand::ElementClick(element_id, reply) => {
                 webdriver_handlers::handle_element_click(
                     &documents,

components/script/webdriver_handlers.rs

@@ -843,6 +843,27 @@ pub(crate) fn handle_find_element_elements_tag_name(
         .unwrap();
 }
 
+/// <https://www.w3.org/TR/webdriver2/#dfn-get-element-shadow-root>
+pub(crate) fn handle_get_element_shadow_root(
+    documents: &DocumentCollection,
+    pipeline: PipelineId,
+    element_id: String,
+    reply: IpcSender<Result<Option<String>, ErrorStatus>>,
+) {
+    reply
+        .send(
+            find_node_by_unique_id(documents, pipeline, element_id).and_then(|node| match node
+                .downcast::<Element>(
+            ) {
+                Some(element) => Ok(element
+                    .GetShadowRoot()
+                    .map(|x| x.upcast::<Node>().unique_id(pipeline))),
+                None => Err(ErrorStatus::NoSuchElement),
+            }),
+        )
+        .unwrap();
+}
+
 pub(crate) fn handle_will_send_keys(
     documents: &DocumentCollection,
     pipeline: PipelineId,

components/shared/embedder/webdriver.rs

@@ -130,6 +130,7 @@ pub enum WebDriverScriptCommand {
         IpcSender<Result<Vec<String>, ErrorStatus>>,
     ),
     FindElementElementsTagName(String, String, IpcSender<Result<Vec<String>, ErrorStatus>>),
+    GetElementShadowRoot(String, IpcSender<Result<Option<String>, ErrorStatus>>),
     ElementClick(String, IpcSender<Result<Option<String>, ErrorStatus>>),
     GetActiveElement(IpcSender<Option<String>>),
     GetComputedRole(String, IpcSender<Result<Option<String>, ErrorStatus>>),

components/webdriver_server/lib.rs

@@ -1214,6 +1214,28 @@ impl Handler {
         }
     }
 
+    fn handle_get_shadow_root(&self, element: WebElement) -> WebDriverResult<WebDriverResponse> {
+        let (sender, receiver) = ipc::channel().unwrap();
+        let cmd = WebDriverScriptCommand::GetElementShadowRoot(element.to_string(), sender);
+        self.browsing_context_script_command(cmd)?;
+        match wait_for_script_response(receiver)? {
+            Ok(value) => {
+                if value.is_none() {
+                    return Err(WebDriverError::new(
+                        ErrorStatus::NoSuchShadowRoot,
+                        "No shadow root found for the element",
+                    ));
+                }
+                let value_resp = serde_json::to_value(
+                    value.map(|x| serde_json::to_value(WebElement(x)).unwrap()),
+                )?;
+                let shadow_root_value = json!({ SHADOW_ROOT_IDENTIFIER: value_resp });
+                Ok(WebDriverResponse::Generic(ValueResponse(shadow_root_value)))
+            },
+            Err(error) => Err(WebDriverError::new(error, "")),
+        }
+    }
+
     // https://w3c.github.io/webdriver/webdriver-spec.html#get-element-rect
     fn handle_element_rect(&self, element: &WebElement) -> WebDriverResult<WebDriverResponse> {
         let (sender, receiver) = ipc::channel().unwrap();
@@ -1936,6 +1958,7 @@ impl WebDriverHandler<ServoExtensionRoute> for Handler {
             WebDriverCommand::FindElementElements(ref element, ref parameters) => {
                 self.handle_find_elements_from_element(element, parameters)
             },
+            WebDriverCommand::GetShadowRoot(element) => self.handle_get_shadow_root(element),
             WebDriverCommand::GetNamedCookie(name) => self.handle_get_cookie(name),
             WebDriverCommand::GetCookies => self.handle_get_cookies(),
             WebDriverCommand::GetActiveElement => self.handle_active_element(),