A MessageObject is a purely internal object to hold information about

src/objects-visiting.cc

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 86 matching lines @@
       return kVisitDataObjectGeneric;
 
     case JS_OBJECT_TYPE:
     case JS_CONTEXT_EXTENSION_OBJECT_TYPE:
     case JS_VALUE_TYPE:
     case JS_ARRAY_TYPE:
     case JS_REGEXP_TYPE:
     case JS_GLOBAL_PROXY_TYPE:
     case JS_GLOBAL_OBJECT_TYPE:
     case JS_BUILTINS_OBJECT_TYPE:
+    case JS_MESSAGE_OBJECT_TYPE:
       return GetVisitorIdForSize(kVisitJSObject,
                                  kVisitJSObjectGeneric,
                                  instance_size);
 
     case JS_FUNCTION_TYPE:
       return kVisitJSFunction;
 
     case HEAP_NUMBER_TYPE:
     case PIXEL_ARRAY_TYPE:
     case EXTERNAL_BYTE_ARRAY_TYPE:
@@ skipping 15 matching lines @@
                                      kVisitStructGeneric,
                                      instance_size);
 
     default:
       UNREACHABLE();
       return kVisitorIdCount;
   }
 }
 
 } }  // namespace v8::internal