LibWeb: Start implementing the Element scroll attributes

These are required for hit testing the document in Google Docs. If they
aren't defined, the Google Docs hit test code will add undefined to
certain values, causing them to turn into NaN. This causes NaNs to
propagate through their hit test code, which eventually makes it
infinitely loop.

Userland/Libraries/LibWeb/DOM/Document.h

@@ -298,6 +298,7 @@ public:
     WebIDL::ExceptionOr<void> close();
 
     HTML::Window* default_view() { return m_window.ptr(); }
+    HTML::Window const* default_view() const { return m_window.ptr(); }
 
     String const& content_type() const { return m_content_type; }
     void set_content_type(String const& content_type) { m_content_type = content_type; }