Add NUM_VALUES for enums and size for structs (see #3836)

API versioning requires that enumerations end with a count value
(`*_NUM_VALUES`) and structs begin with a size value (`size_t size`).
Wrapper templates are updated to support structs with different size
values indicating different versions.

To test:
Run `ceftests --gtest_filter=ApiVersionTest.StructVersion*`

tests/cefclient/browser/dialog_handler_gtk.cc

@@ -393,6 +393,10 @@ void ClientDialogHandlerGtk::OnJSDialogContinue(const OnJSDialogParams& params,
       gtk_message_type = GTK_MESSAGE_QUESTION;
       title = "JavaScript Prompt";
       break;
+
+    case JSDIALOGTYPE_NUM_VALUES:
+      NOTREACHED();
+      return;
   }
 
   js_dialog_callback_ = params.callback;