LibWeb: Make CSSStyleDeclaration GC-allocated

Author: https://github.com/awesomekling
Commit: 72bacba97b
Pull-request: https://github.com/SerenityOS/serenity/pull/14816
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/HTML/Window.cpp

@@ -319,7 +319,7 @@ Page const* Window::page() const
     return associated_document().page();
 }
 
-NonnullRefPtr<CSS::CSSStyleDeclaration> Window::get_computed_style(DOM::Element& element) const
+CSS::CSSStyleDeclaration* Window::get_computed_style(DOM::Element& element) const
 {
     return CSS::ResolvedCSSStyleDeclaration::create(element);
 }