Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/factory.h

 // 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 298 matching lines @@
                            PretenureFlag pretenure = NOT_TENURED);
 
   Handle<Object> NewNumberFromInt(int32_t value,
                                   PretenureFlag pretenure = NOT_TENURED);
   Handle<Object> NewNumberFromUint(uint32_t value,
                                   PretenureFlag pretenure = NOT_TENURED);
   inline Handle<Object> NewNumberFromSize(size_t value,
                                    PretenureFlag pretenure = NOT_TENURED);
   Handle<HeapNumber> NewHeapNumber(double value,
                                    PretenureFlag pretenure = NOT_TENURED);
+  Handle<HeapNumber> NewMutableHeapNumber(
+      double value,
+      PretenureFlag pretenure = NOT_TENURED);
 
 
   // These objects are used by the api to create env-independent data
   // structures in the heap.
   Handle<JSObject> NewNeanderObject();
 
   Handle<JSObject> NewArgumentsObject(Handle<Object> callee, int length);
 
   // JS objects are pretenured when allocated by the bootstrapper and
   // runtime.
@@ skipping 296 matching lines @@
                           isolate());
   } else {
     return NewNumber(static_cast<double>(value), pretenure);
   }
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_FACTORY_H_