Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/mark-compact.cc

 // Copyright 2012 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 296 matching lines @@
           case TYPE_FEEDBACK_INFO_TYPE:
             object->Iterate(this);
             break;
           case DECLARED_ACCESSOR_INFO_TYPE:
           case EXECUTABLE_ACCESSOR_INFO_TYPE:
           case BYTE_ARRAY_TYPE:
           case CALL_HANDLER_INFO_TYPE:
           case CODE_TYPE:
           case FIXED_DOUBLE_ARRAY_TYPE:
           case HEAP_NUMBER_TYPE:
+          case MUTABLE_HEAP_NUMBER_TYPE:
           case INTERCEPTOR_INFO_TYPE:
           case ODDBALL_TYPE:
           case SCRIPT_TYPE:
           case SHARED_FUNCTION_INFO_TYPE:
             break;
           default:
             UNREACHABLE();
         }
       }
     }
@@ skipping 4193 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal