[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,