Introduce toHTMLInputElement(Node*), and use it.

Source/core/rendering/RenderImage.cpp

Index: Source/core/rendering/RenderImage.cpp
diff --git a/Source/core/rendering/RenderImage.cpp b/Source/core/rendering/RenderImage.cpp
index 9b3059913daa2211381c5854690b3226be7a08eb..75fa0d800d09d9f0717bc0dba4d69a8ed9cd5fe6 100644
--- a/Source/core/rendering/RenderImage.cpp
+++ b/Source/core/rendering/RenderImage.cpp
@@ -526,7 +526,7 @@ void RenderImage::updateAltText()
         return;
 
     if (node()->hasTagName(inputTag))
-        m_altText = static_cast<HTMLInputElement*>(node())->altText();
+        m_altText = toHTMLInputElement(node())->altText();
     else if (node()->hasTagName(imgTag))
         m_altText = static_cast<HTMLImageElement*>(node())->altText();
 }