LibWeb: Implement (naive) version of HTMLIFrameElement.contentWindow

This should really return the WindowProxy, but since we don't have the
infrastructure set up just yet, just return the window object itself
for now.

Userland/Libraries/LibWeb/HTML/BrowsingContextContainer.cpp

@@ -103,4 +103,13 @@ const DOM::Document* BrowsingContextContainer::get_svg_document() const
     return nullptr;
 }
 
+HTML::Window* BrowsingContextContainer::content_window() const
+{
+    // FIXME: This should return the WindowProxy
+    auto* document = content_document();
+    if (!document)
+        return nullptr;
+    return const_cast<HTML::Window*>(&document->window());
+}
+
 }