LibWeb: Move ad hoc CallbackType helper method to CallbackType header

Abstract operations of a stream does not seem like the correct home for
this function.

Libraries/LibWeb/WebIDL/CallbackType.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/VM.h>
 #include <LibWeb/HTML/Scripting/Environments.h>
 #include <LibWeb/WebIDL/CallbackType.h>
 
@@ -26,4 +27,19 @@ void CallbackType::visit_edges(Cell::Visitor& visitor)
     visitor.visit(callback_context);
 }
 
+// Non-standard function to aid in converting a user-provided function into a WebIDL::Callback. This is essentially
+// what the Bindings generator would do at compile time, but at runtime instead.
+JS::ThrowCompletionOr<GC::Root<CallbackType>> property_to_callback(JS::VM& vm, JS::Value value, JS::PropertyKey const& property_key, OperationReturnsPromise operation_returns_promise)
+{
+    auto property = TRY(value.get(vm, property_key));
+
+    if (property.is_undefined())
+        return GC::Root<CallbackType> {};
+
+    if (!property.is_function())
+        return vm.throw_completion<JS::TypeError>(JS::ErrorType::NotAFunction, property.to_string_without_side_effects());
+
+    return vm.heap().allocate<CallbackType>(property.as_object(), HTML::incumbent_realm(), operation_returns_promise);
+}
+
 }

Libraries/LibWeb/WebIDL/CallbackType.h

@@ -39,4 +39,6 @@ private:
     virtual void visit_edges(Cell::Visitor&) override;
 };
 
+JS::ThrowCompletionOr<GC::Root<WebIDL::CallbackType>> property_to_callback(JS::VM& vm, JS::Value value, JS::PropertyKey const& property_key, WebIDL::OperationReturnsPromise);
+
 }