Introduce toHTMLInputElement(Node*), and use it.

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 3ba55d4c0a82b4326a3b709220331a2c4dde724a..5398661a4b76e7e814b7f81b3000c8721a52f9d6 100644
--- a/Source/core/rendering/RenderButton.cpp
+++ b/Source/core/rendering/RenderButton.cpp
@@ -109,7 +109,7 @@ void RenderButton::updateFromElement()
 {
     // If we're an input element, we may need to change our button text.
     if (node()->hasTagName(inputTag)) {
-        HTMLInputElement* input = static_cast<HTMLInputElement*>(node());
+        HTMLInputElement* input = toHTMLInputElement(node());
         String value = input->valueWithDefault();
         setText(value);
     }