Make WebNode::to<Type>() (and toConst) casts contain type asserts.

third_party/WebKit/public/web/WebFormElement.h

Index: third_party/WebKit/public/web/WebFormElement.h
diff --git a/third_party/WebKit/public/web/WebFormElement.h b/third_party/WebKit/public/web/WebFormElement.h
index 14137a49ec3a5880c3144d3d5561ea3eed738913..35fac53585e316450c565fe9a20deccce51462f7 100644
--- a/third_party/WebKit/public/web/WebFormElement.h
+++ b/third_party/WebKit/public/web/WebFormElement.h
@@ -41,7 +41,7 @@ class WebFormControlElement;
 
 // A container for passing around a reference to a form element. Provides some
 // information about the form.
-class WebFormElement : public WebElement {
+class WebFormElement final : public WebElement {
 public:
     ~WebFormElement() { reset(); }
 
@@ -83,6 +83,8 @@ public:
 #endif
 };
 
+DECLARE_WEB_NODE_TYPE_CASTS(WebFormElement);
+
 } // namespace blink
 
 #endif