::backdrop pseudo element should react to event listeners

[Tim Nguyen (:ntim)]

```
<dialog id=dialog onclick="alert('clicked')">hello</dialog>

<script>
dialog.showModal()
</script>
```

the onclick does not work on the ::backdrop, it does in Firefox/Chrome.

[Radar WebKit Bug Importer]

<rdar://problem/85359803>

[Tim Nguyen (:ntim)]

WPT: https://github.com/web-platform-tests/wpt/blob/master/html/semantics/interactive-elements/the-dialog-element/backdrop-receives-element-events.html

[Tim Nguyen (:ntim)]

Created attachment 448789 [details]
Patch

[zalan]

Comment on attachment 448789 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=448789&action=review

> Source/WebCore/rendering/updating/RenderTreeUpdaterGeneratedContent.cpp:201
> -    // Update or attach to renderer parent
> +    // Update or attach to renderer
>      WeakPtr currentParent = backdropRenderer->parent();
> -    WeakPtr newParent = renderer.parent();
> +    WeakPtr newParent = renderer;

auto backdropRenderer = renderer.backdropRenderer();
   ...
    renderer.setBackdropRenderer(*backdropRenderer);

don't we always end up either with no backdropRenderer or with one with the correct parent (renderer)? I didn't look through all the cases but could you explain the re-parenting case? (if no reparenting ever happens, we could make this function a lot simpler)

[Tim Nguyen (:ntim)]

Created attachment 448843 [details]
Patch

[EWS]

Committed r287878 (245922@main): <https://commits.webkit.org/245922@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 448843 [details].