Introduce short external strings without pointer cache.

src/objects-visiting.cc

 // Copyright 2011 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 46 matching lines @@
         } else {
           return kVisitConsString;
         }
 
       case kSlicedStringTag:
         return kVisitSlicedString;
 
       case kExternalStringTag:
         return GetVisitorIdForSize(kVisitDataObject,
                                    kVisitDataObjectGeneric,
-                                   ExternalString::kSize);
+                                   instance_size);
     }
     UNREACHABLE();
   }
 
   switch (instance_type) {
     case BYTE_ARRAY_TYPE:
       return kVisitByteArray;
 
     case FREE_SPACE_TYPE:
       return kVisitFreeSpace;
@@ skipping 90 matching lines @@
                                      kVisitStructGeneric,
                                      instance_size);
 
     default:
       UNREACHABLE();
       return kVisitorIdCount;
   }
 }
 
 } }  // namespace v8::internal