Add const versions of some toHTMLFooElement().

Source/core/html/FormAssociatedElement.cpp

Index: Source/core/html/FormAssociatedElement.cpp
diff --git a/Source/core/html/FormAssociatedElement.cpp b/Source/core/html/FormAssociatedElement.cpp
index 4f68861f36378a2e00c03a31cc0a310188b5f100..e800826d4fb7ac47a934c4b5717ec454268ce4c8 100644
--- a/Source/core/html/FormAssociatedElement.cpp
+++ b/Source/core/html/FormAssociatedElement.cpp
@@ -277,11 +277,9 @@ bool FormAssociatedElement::isFormControlElementWithState() const
 const HTMLElement* toHTMLElement(const FormAssociatedElement* associatedElement)
 {
     if (associatedElement->isFormControlElement())
-        return static_cast<const HTMLFormControlElement*>(associatedElement);
+        return toHTMLFormControlElement(associatedElement);
     // Assumes the element is an HTMLObjectElement
-    const HTMLElement* element = static_cast<const HTMLObjectElement*>(associatedElement);
-    ASSERT(element->hasTagName(objectTag));
-    return element;
+    return toHTMLObjectElement(associatedElement);
 }
 
 HTMLElement* toHTMLElement(FormAssociatedElement* associatedElement)