LibWebView: Define the OOPWV's superclass in one place

This lets us change it later without having to remember to update a
bunch of uses. (More are coming in the next commit.)

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -26,6 +26,8 @@ class OutOfProcessWebView final
     , public ViewImplementation {
     C_OBJECT(OutOfProcessWebView);
 
+    using Super = GUI::AbstractScrollableWidget;
+
 public:
     virtual ~OutOfProcessWebView() override;