WebContent+Friends: Add IPC and plumbing for WebDriver endpoint

To use the `GET /session/{id}/element/{id}/css/{property name}`
WebDriver endpoint, two new IPC calls through the Browser are
implemented:
    - get_active_documents_type returns the type of the active document,
      which is either "xml" or "html"
    - get_computed_value_for_element returns the computed CSS value (as
      String) for the given element and CSS property name

Userland/Services/WebContent/ConnectionFromClient.cpp

@@ -18,6 +18,7 @@
 #include <LibJS/Parser.h>
 #include <LibJS/Runtime/ConsoleObject.h>
 #include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/CSS/PropertyID.h>
 #include <LibWeb/Cookie/ParsedCookie.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/NodeList.h>
@@ -517,6 +518,49 @@ Messages::WebContentServer::GetElementPropertyResponse ConnectionFromClient::get
     return { string_or_error.release_value() };
 }
 
+Messages::WebContentServer::GetActiveDocumentsTypeResponse ConnectionFromClient::get_active_documents_type()
+{
+    auto* active_document = page().top_level_browsing_context().active_document();
+
+    if (!active_document)
+        return { "" };
+
+    auto type = active_document->document_type();
+
+    switch (type) {
+    case Web::DOM::Document::Type::HTML:
+        return { "html" };
+        break;
+    case Web::DOM::Document::Type::XML:
+        return { "xml" };
+        break;
+    }
+
+    return { "" };
+}
+
+Messages::WebContentServer::GetComputedValueForElementResponse ConnectionFromClient::get_computed_value_for_element(i32 element_id, String const& property_name)
+{
+    auto* node = Web::DOM::Node::from_id(element_id);
+    if (!node)
+        return { "" };
+
+    if (!node->is_element())
+        return { "" };
+
+    auto& element = verify_cast<Web::DOM::Element>(*node);
+
+    auto property_id = Web::CSS::property_id_from_string(property_name);
+
+    auto computed_values = element.computed_css_values();
+    if (!computed_values)
+        return { "" };
+
+    auto style_value = computed_values->property(property_id);
+
+    return { style_value->to_string() };
+}
+
 Messages::WebContentServer::GetSelectedTextResponse ConnectionFromClient::get_selected_text()
 {
     return page().focused_context().selected_text();