Introduce toHTMLFormControlElement, and use it.

Source/core/loader/FormSubmission.cpp

Index: Source/core/loader/FormSubmission.cpp
diff --git a/Source/core/loader/FormSubmission.cpp b/Source/core/loader/FormSubmission.cpp
index 22bc20c5a715bb03280902e2c7bac10dd52d87cc..09bfb9e2704fd8ad83da589640c30d99c686b9f8 100644
--- a/Source/core/loader/FormSubmission.cpp
+++ b/Source/core/loader/FormSubmission.cpp
@@ -142,7 +142,7 @@ PassRefPtr<FormSubmission> FormSubmission::create(HTMLFormElement* form, const A
     if (event && event->target()) {
         for (Node* node = event->target()->toNode(); node; node = node->parentNode()) {
             if (node->isElementNode() && toElement(node)->isFormControlElement()) {
-                submitButton = static_cast<HTMLFormControlElement*>(node);
+                submitButton = toHTMLFormControlElement(node);
                 break;
             }
         }