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

third_party/WebKit/public/web/WebDocumentType.h

Index: third_party/WebKit/public/web/WebDocumentType.h
diff --git a/third_party/WebKit/public/web/WebDocumentType.h b/third_party/WebKit/public/web/WebDocumentType.h
index e3f33f54bd388d8b829bc90912189372e506b015..0ae0948e836a8609add2e64ec54390ae897f881a 100644
--- a/third_party/WebKit/public/web/WebDocumentType.h
+++ b/third_party/WebKit/public/web/WebDocumentType.h
@@ -43,7 +43,7 @@ class DocumentType;
 class WebString;
 
 // Provides readonly access to some properties of a DOM document type.
-class WebDocumentType : public WebNode {
+class WebDocumentType final : public WebNode {
 public:
     WebDocumentType() { }
     WebDocumentType(const WebDocumentType& e) : WebNode(e) { }
@@ -64,6 +64,8 @@ public:
 #endif
 };
 
+DECLARE_WEB_NODE_TYPE_CASTS(WebDocumentType);
+
 } // namespace blink
 
 #endif