LibWeb: Expose the Window object as Document.defaultView

This should really be a WindowProxy? but since we don't have anything
representing that concept yet, let's just expose the Window object
directly so document.defaultView.foo works. :^)

Userland/Libraries/LibWeb/DOM/Document.h

@@ -214,6 +214,8 @@ public:
 
     Window& window() { return *m_window; }
 
+    Window* default_view() { return m_window; }
+
     const String& content_type() const { return m_content_type; }
     void set_content_type(const String& content_type) { m_content_type = content_type; }
 

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -9,6 +9,8 @@ interface Document : Node {
     readonly attribute DOMString inputEncoding;
     readonly attribute DOMString contentType;
 
+    readonly attribute Window? defaultView;
+
     Element? getElementById(DOMString id);
     ArrayFromVector getElementsByName(DOMString name);
     ArrayFromVector getElementsByTagName(DOMString tagName);