Cpp bindings: Return false from Validator::Accept() on unrecognized message or invalid flags

mojo/public/cpp/bindings/lib/validation_errors.h

Index: mojo/public/cpp/bindings/lib/validation_errors.h
diff --git a/mojo/public/cpp/bindings/lib/validation_errors.h b/mojo/public/cpp/bindings/lib/validation_errors.h
index fbba6b30488913dcd9d5f09e16066835970748fd..0d38e79e25359dec9f01b5b32c8b7bcc80b59069 100644
--- a/mojo/public/cpp/bindings/lib/validation_errors.h
+++ b/mojo/public/cpp/bindings/lib/validation_errors.h
@@ -48,7 +48,9 @@ enum ValidationError {
   // lengths.
   VALIDATION_ERROR_DIFFERENT_SIZED_ARRAYS_IN_MAP,
   // Attempted to deserialize a tagged union with an unknown tag.
-  VALIDATION_ERROR_UNKOWN_UNION_TAG
+  VALIDATION_ERROR_UNKOWN_UNION_TAG,
+  // The |name| field in a message header contains an unexpected value.
+  VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD
 };
 
 const char* ValidationErrorToString(ValidationError error);