Fix assertion failure in InteractionRegion::isOverlay

https://bugs.webkit.org/show_bug.cgi?id=259598
<rdar://113037867>

Reviewed by Tim Horton.

We're not supposed to use `firstChildBox()` if the first child might be
of another type. Using `firstChild()` directly instead.

* Source/WebCore/page/InteractionRegion.cpp:
(WebCore::isOverlay):

Canonical link: https://commits.webkit.org/266395@main

Source/WebCore/page/InteractionRegion.cpp

@@ -176,7 +176,7 @@ static bool isOverlay(const RenderElement& renderer)
         auto lastRenderer = renderBox;
         for (auto& ancestor : ancestorsOfType<RenderBox>(renderer)) {
             // We don't want to occlude any previous siblings.
-            if (ancestor.firstChildBox() != lastRenderer)
+            if (ancestor.firstChild() != lastRenderer)
                 return false;
             lastRenderer = &ancestor;
             if (ancestor.absoluteContentBox() != refContentBox)