Add toHTMLSummaryElement(), and use it.

Source/core/html/shadow/DetailsMarkerControl.cpp

Index: Source/core/html/shadow/DetailsMarkerControl.cpp
diff --git a/Source/core/html/shadow/DetailsMarkerControl.cpp b/Source/core/html/shadow/DetailsMarkerControl.cpp
index 77f6b7cd147d09eb4ba76eccf2a114eb997b3721..ed9d7394b930b14be2a6382366299c4048ec5a50 100644
--- a/Source/core/html/shadow/DetailsMarkerControl.cpp
+++ b/Source/core/html/shadow/DetailsMarkerControl.cpp
@@ -56,9 +56,7 @@ bool DetailsMarkerControl::rendererIsNeeded(const NodeRenderingContext& context)
 
 HTMLSummaryElement* DetailsMarkerControl::summaryElement()
 {
-    Element* element = shadowHost();
-    ASSERT_WITH_SECURITY_IMPLICATION(!element || element->hasTagName(summaryTag));
-    return static_cast<HTMLSummaryElement*>(element);
+    return toHTMLSummaryElement(shadowHost());
 }
 
 }