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

third_party/WebKit/public/web/WebPluginDocument.h

Index: third_party/WebKit/public/web/WebPluginDocument.h
diff --git a/third_party/WebKit/public/web/WebPluginDocument.h b/third_party/WebKit/public/web/WebPluginDocument.h
index 7424bae7a2f7b8e0547dda551a793cc6d9bc0855..44683519173037275dadf1be2bb3733547816b88 100644
--- a/third_party/WebKit/public/web/WebPluginDocument.h
+++ b/third_party/WebKit/public/web/WebPluginDocument.h
@@ -39,7 +39,7 @@ class PluginDocument;
 class WebPlugin;
 
 // Wraps a WebDocument for full page plugins.
-class WebPluginDocument : public WebDocument {
+class WebPluginDocument final : public WebDocument {
 public:
     WebPluginDocument() { }
     WebPluginDocument(const WebPluginDocument& e) : WebDocument(e) { }
@@ -60,6 +60,8 @@ public:
 #endif
 };
 
+DECLARE_WEB_NODE_TYPE_CASTS(WebPluginDocument);
+
 } // namespace blink
 
 #endif