UI: Set headless-browser width and height

Author: https://github.com/shlyakpavel
Commit: ce56bc29e2
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2323
Reviewed-by: https://github.com/AtkinsSJ

UI/Headless/Application.h

@@ -66,6 +66,8 @@ public:
     bool rebaseline { false };
     bool verbose { false };
     int per_test_timeout_in_seconds { 30 };
+    int width { 800 };
+    int height { 600 };
 
 private:
     RefPtr<Requests::RequestClient> m_request_client;