Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/heap-inl.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 255 matching lines @@
     profiler->AllocationEvent(object->address(), size_in_bytes);
   }
   return result;
 }
 
 
 MaybeObject* Heap::NumberFromInt32(
     int32_t value, PretenureFlag pretenure) {
   if (Smi::IsValid(value)) return Smi::FromInt(value);
   // Bypass NumberFromDouble to avoid various redundant checks.
-  return AllocateHeapNumber(FastI2D(value), pretenure);
+  return AllocateHeapNumber(FastI2D(value), IMMUTABLE, pretenure);
 }
 
 
 MaybeObject* Heap::NumberFromUint32(
     uint32_t value, PretenureFlag pretenure) {
   if (static_cast<int32_t>(value) >= 0 &&
       Smi::IsValid(static_cast<int32_t>(value))) {
     return Smi::FromInt(static_cast<int32_t>(value));
   }
   // Bypass NumberFromDouble to avoid various redundant checks.
-  return AllocateHeapNumber(FastUI2D(value), pretenure);
+  return AllocateHeapNumber(FastUI2D(value), IMMUTABLE, pretenure);
 }
 
 
 void Heap::FinalizeExternalString(String* string) {
   ASSERT(string->IsExternalString());
   v8::String::ExternalStringResourceBase** resource_addr =
       reinterpret_cast<v8::String::ExternalStringResourceBase**>(
           reinterpret_cast<byte*>(string) +
           ExternalString::kResourceOffset -
           kHeapObjectTag);
@@ skipping 534 matching lines @@
 
 
 double GCTracer::SizeOfHeapObjects() {
   return (static_cast<double>(heap_->SizeOfObjects())) / MB;
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_INL_H_