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

third_party/WebKit/public/web/WebTextAreaElement.h

Index: third_party/WebKit/public/web/WebTextAreaElement.h
diff --git a/third_party/WebKit/public/web/WebTextAreaElement.h b/third_party/WebKit/public/web/WebTextAreaElement.h
index 04b5e736808085ab3ed9e0c88bc003862cd39f88..83c543ef0a2720c010ff70983f745f3155cd7b03 100644
--- a/third_party/WebKit/public/web/WebTextAreaElement.h
+++ b/third_party/WebKit/public/web/WebTextAreaElement.h
@@ -38,7 +38,7 @@ namespace blink {
 class HTMLTextAreaElement;
 
 // Provides access to some properties of a DOM textarea element node.
-class WebTextAreaElement : public WebFormControlElement {
+class WebTextAreaElement final : public WebFormControlElement {
 public:
     WebTextAreaElement() : WebFormControlElement() { }
     WebTextAreaElement(const WebTextAreaElement& element) : WebFormControlElement(element) { }
@@ -57,6 +57,8 @@ public:
 #endif
 };
 
+DECLARE_WEB_NODE_TYPE_CASTS(WebTextAreaElement);
+
 } // namespace blink
 
 #endif