Regression: Add back support for hr elements in select listbox

Source/core/html/HTMLSelectElement.cpp

Index: Source/core/html/HTMLSelectElement.cpp
diff --git a/Source/core/html/HTMLSelectElement.cpp b/Source/core/html/HTMLSelectElement.cpp
index 82da0779b342a0275c7bf979e95f88268c8f16bc..d44db4e8a8612bc7bdb78ff28c9f57618c271100 100644
--- a/Source/core/html/HTMLSelectElement.cpp
+++ b/Source/core/html/HTMLSelectElement.cpp
@@ -1763,7 +1763,7 @@ void HTMLSelectElement::trace(Visitor* visitor)
 void HTMLSelectElement::didAddUserAgentShadowRoot(ShadowRoot& root)
 {
     RefPtrWillBeRawPtr<HTMLContentElement> content = HTMLContentElement::create(document());
-    content->setAttribute(selectAttr, "option,optgroup");
+    content->setAttribute(selectAttr, "option,optgroup,hr");
     root.appendChild(content);
 }