LibWeb: Use browsing context creator URL for about:blank documents

In about:blank documents, we should use the browsing context's creator
URL as the base URL, if it exists and there is no <base> element.
This means that any about:blank frames will have URLs parse relative to
their parent frame's URL.

Fixes #17394.

Userland/Libraries/LibWeb/HTML/BrowsingContext.h

@@ -265,6 +265,8 @@ public:
     // https://html.spec.whatwg.org/multipage/window-object.html#close-a-browsing-context
     void close();
 
+    Optional<AK::URL> const& creator_url() const { return m_creator_url; }
+
 private:
     explicit BrowsingContext(Page&, HTML::BrowsingContextContainer*);