Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/cpu-profiler.h"
@@ skipping 274 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 4183 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal