Use new is*Element() helper functions more in rendering code

Source/core/rendering/RenderButton.cpp

Index: Source/core/rendering/RenderButton.cpp
diff --git a/Source/core/rendering/RenderButton.cpp b/Source/core/rendering/RenderButton.cpp
index ce1bec473cea433db1a8540764293a65700f8164..76accbfb3388247eeb63f24af9ea466f3b9c983c 100644
--- a/Source/core/rendering/RenderButton.cpp
+++ b/Source/core/rendering/RenderButton.cpp
@@ -104,7 +104,7 @@ bool RenderButton::canHaveGeneratedChildren() const
     // Input elements can't have generated children, but button elements can. We'll
     // write the code assuming any other button types that might emerge in the future
     // can also have children.
-    return !node()->hasTagName(inputTag);
+    return !isHTMLInputElement(*node());
 }
 
 LayoutRect RenderButton::controlClipRect(const LayoutPoint& additionalOffset) const