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

Source/core/rendering/RenderView.cpp

Index: Source/core/rendering/RenderView.cpp
diff --git a/Source/core/rendering/RenderView.cpp b/Source/core/rendering/RenderView.cpp
index bbb9ce50e38269377d5167f7072fd73250449716..ec0f512e64f6378c6eea3a16046e213d6fb304a0 100644
--- a/Source/core/rendering/RenderView.cpp
+++ b/Source/core/rendering/RenderView.cpp
@@ -157,7 +157,7 @@ void RenderView::positionDialogs()
     TrackedRendererListHashSet::iterator end = positionedDescendants->end();
     for (TrackedRendererListHashSet::iterator it = positionedDescendants->begin(); it != end; ++it) {
         RenderBox* box = *it;
-        if (box->node() && box->node()->hasTagName(HTMLNames::dialogTag))
+        if (isHTMLDialogElement(box->node()))
             positionDialog(box);
     }
 }