LibWeb: Implement getting the top-level traversable of a navigable

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -95,4 +95,18 @@ JS::GCPtr<TraversableNavigable> Navigable::traversable_navigable()
     return static_cast<TraversableNavigable*>(navigable);
 }
 
+// https://html.spec.whatwg.org/multipage/document-sequences.html#nav-top
+JS::GCPtr<TraversableNavigable> Navigable::top_level_traversable()
+{
+    // 1. Let navigable be inputNavigable.
+    auto navigable = this;
+
+    // 2. While navigable's parent is not null, set navigable to navigable's parent.
+    while (navigable->parent())
+        navigable = navigable->parent();
+
+    // 3. Return navigable.
+    return verify_cast<TraversableNavigable>(navigable);
+}
+
 }

Userland/Libraries/LibWeb/HTML/Navigable.h

@@ -41,6 +41,7 @@ public:
     void set_container(JS::GCPtr<NavigableContainer>);
 
     JS::GCPtr<TraversableNavigable> traversable_navigable();
+    JS::GCPtr<TraversableNavigable> top_level_traversable();
 
 protected:
     Navigable();