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/HTML/HTMLMediaElement.cpp

@@ -13,7 +13,7 @@ namespace Web::HTML {
 HTMLMediaElement::HTMLMediaElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : HTMLElement(document, move(qualified_name))
 {
-    set_prototype(&window().ensure_web_prototype<Bindings::HTMLMediaElementPrototype>("HTMLMediaElement"));
+    set_prototype(&window().cached_web_prototype("HTMLMediaElement"));
 }
 
 HTMLMediaElement::~HTMLMediaElement() = default;