LibWeb: Make Document::url() return URL by value

Returning it by reference can lead to unpleasant situations if we use
this getter when the document may go away. Better to make the getter
return a copy than have to think about this everywhere.

Libraries/LibWeb/DOM/Document.h

@@ -53,7 +53,7 @@ public:
     virtual ~Document() override;
 
     void set_url(const URL& url) { m_url = url; }
-    const URL& url() const { return m_url; }
+    URL url() const { return m_url; }
 
     Origin origin() const;