cefclient: Fix invalid cast to ClientHandlerStd (see #3499)

tests/cefclient/browser/default_client_handler.cc

@@ -0,0 +1,19 @@
+// Copyright (c) 2024 The Chromium Embedded Framework Authors. All rights
+// reserved. Use of this source code is governed by a BSD-style license that
+// can be found in the LICENSE file.
+
+#include "tests/cefclient/browser/default_client_handler.h"
+
+namespace client {
+
+// static
+CefRefPtr<DefaultClientHandler> DefaultClientHandler::GetForClient(
+    CefRefPtr<CefClient> client) {
+  auto base = BaseClientHandler::GetForClient(client);
+  if (base && base->GetTypeKey() == &kTypeKey) {
+    return static_cast<DefaultClientHandler*>(base.get());
+  }
+  return nullptr;
+}
+
+}  // namespace client