Auto merge of #23008 - georgeroman:implement_webdriver_find_elem_from_elem, r=Manishearth

Implement WebDriver FindElementFromElement command

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components/script/script_thread.rs

@@ -1859,6 +1859,15 @@ impl ScriptThread {
                     reply,
                 )
             },
+            WebDriverScriptCommand::FindElementElementCSS(selector, element_id, reply) => {
+                webdriver_handlers::handle_find_element_element_css(
+                    &*documents,
+                    pipeline_id,
+                    element_id,
+                    selector,
+                    reply,
+                )
+            },
             WebDriverScriptCommand::FocusElement(element_id, reply) => {
                 webdriver_handlers::handle_focus_element(
                     &*documents,

components/script/webdriver_handlers.rs

@@ -203,6 +203,23 @@ pub fn handle_find_elements_css(
     reply.send(node_ids).unwrap();
 }
 
+pub fn handle_find_element_element_css(
+    documents: &Documents,
+    pipeline: PipelineId,
+    element_id: String,
+    selector: String,
+    reply: IpcSender<Result<Option<String>, ()>>,
+) {
+    let node_id = find_node_by_unique_id(documents, pipeline, element_id)
+        .ok_or(())
+        .and_then(|node| {
+            node.query_selector(DOMString::from(selector))
+                .map_err(|_| ())
+        })
+        .map(|node| node.map(|x| x.upcast::<Node>().unique_id()));
+    reply.send(node_id).unwrap();
+}
+
 pub fn handle_focus_element(
     documents: &Documents,
     pipeline: PipelineId,