LibWeb: Use cached_web_prototype() as much as possible

Unlike ensure_web_prototype<T>(), the cached version doesn't require the
prototype type to be fully formed, so we can use it without including
the FooPrototype.h header. It's also a bit less verbose. :^)

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -51,7 +51,7 @@ XMLHttpRequest::XMLHttpRequest(HTML::Window& window)
     , m_window(window)
     , m_response_type(Bindings::XMLHttpRequestResponseType::Empty)
 {
-    set_prototype(&window.ensure_web_prototype<Bindings::XMLHttpRequestPrototype>("XMLHttpRequest"));
+    set_prototype(&window.cached_web_prototype("XMLHttpRequest"));
 }
 
 XMLHttpRequest::~XMLHttpRequest() = default;