Use new is*Element() helper functions in DOM code

Source/core/dom/ShadowTreeStyleSheetCollection.cpp

Index: Source/core/dom/ShadowTreeStyleSheetCollection.cpp
diff --git a/Source/core/dom/ShadowTreeStyleSheetCollection.cpp b/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
index 6a74608c5ea5a0763d8bd10e0ecda4c116a17014..3e819928f682563f59ad05d4799d7d80e77992bf 100644
--- a/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
+++ b/Source/core/dom/ShadowTreeStyleSheetCollection.cpp
@@ -53,14 +53,14 @@ void ShadowTreeStyleSheetCollection::collectStyleSheets(StyleEngine* engine, Sty
         StyleSheet* sheet = 0;
         CSSStyleSheet* activeSheet = 0;
 
-        if (!node->isHTMLElement() || !node->hasTagName(styleTag))
+        if (!isHTMLStyleElement(*node))
             continue;
 
-        Element* element = toElement(node);
+        HTMLStyleElement* element = toHTMLStyleElement(node);
         const AtomicString& title = element->fastGetAttribute(titleAttr);
         bool enabledViaScript = false;
 
-        sheet = toHTMLStyleElement(node)->sheet();
+        sheet = element->sheet();
         if (sheet && !sheet->disabled() && sheet->isCSSStyleSheet())
             activeSheet = toCSSStyleSheet(sheet);