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

Source/core/css/StyleSheetList.cpp

Index: Source/core/css/StyleSheetList.cpp
diff --git a/Source/core/css/StyleSheetList.cpp b/Source/core/css/StyleSheetList.cpp
index bc6b0df930638eda87328d74902cea1a6434bbfd..38649f33e222f70d46ee8d2070543087ce56f5de 100644
--- a/Source/core/css/StyleSheetList.cpp
+++ b/Source/core/css/StyleSheetList.cpp
@@ -76,9 +76,7 @@ HTMLStyleElement* StyleSheetList::getNamedItem(const AtomicString& name) const
     // But unicity of stylesheet ids is good practice anyway ;)
     // FIXME: We should figure out if we should change this or fix the spec.
     Element* element = m_treeScope->getElementById(name);
-    if (element && element->hasTagName(styleTag))
-        return toHTMLStyleElement(element);
-    return 0;
+    return isHTMLStyleElement(element) ? toHTMLStyleElement(element) : 0;
 }
 
 CSSStyleSheet* StyleSheetList::anonymousNamedGetter(const AtomicString& name)