Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/heap.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 47 matching lines @@
   V(Oddball, the_hole_value, TheHoleValue)                                     \
   V(Oddball, null_value, NullValue)                                            \
   V(Oddball, true_value, TrueValue)                                            \
   V(Oddball, false_value, FalseValue)                                          \
   V(Oddball, uninitialized_value, UninitializedValue)                          \
   V(Map, cell_map, CellMap)                                                    \
   V(Map, global_property_cell_map, GlobalPropertyCellMap)                      \
   V(Map, shared_function_info_map, SharedFunctionInfoMap)                      \
   V(Map, meta_map, MetaMap)                                                    \
   V(Map, heap_number_map, HeapNumberMap)                                       \
+  V(Map, mutable_heap_number_map, MutableHeapNumberMap)                        \
   V(Map, native_context_map, NativeContextMap)                                 \
   V(Map, fixed_array_map, FixedArrayMap)                                       \
   V(Map, code_map, CodeMap)                                                    \
   V(Map, scope_info_map, ScopeInfoMap)                                         \
   V(Map, fixed_cow_array_map, FixedCOWArrayMap)                                \
   V(Map, fixed_double_array_map, FixedDoubleArrayMap)                          \
   V(Map, constant_pool_array_map, ConstantPoolArrayMap)                        \
   V(Object, no_interceptor_result_sentinel, NoInterceptorResultSentinel)       \
   V(Map, hash_table_map, HashTableMap)                                         \
   V(FixedArray, empty_fixed_array, EmptyFixedArray)                            \
@@ skipping 151 matching lines @@
   V(the_hole_value)                       \
   V(null_value)                           \
   V(true_value)                           \
   V(false_value)                          \
   V(uninitialized_value)                  \
   V(cell_map)                             \
   V(global_property_cell_map)             \
   V(shared_function_info_map)             \
   V(meta_map)                             \
   V(heap_number_map)                      \
+  V(mutable_heap_number_map)              \
   V(native_context_map)                   \
   V(fixed_array_map)                      \
   V(code_map)                             \
   V(scope_info_map)                       \
   V(fixed_cow_array_map)                  \
   V(fixed_double_array_map)               \
   V(constant_pool_array_map)              \
   V(no_interceptor_result_sentinel)       \
   V(hash_table_map)                       \
   V(empty_fixed_array)                    \
@@ skipping 855 matching lines @@
 
   // Same as NewNumberFromDouble, but may return a preallocated/immutable
   // number object (e.g., minus_zero_value_, nan_value_)
   MUST_USE_RESULT MaybeObject* NumberFromDouble(
       double value, PretenureFlag pretenure = NOT_TENURED);
 
   // Allocated a HeapNumber from value.
   MUST_USE_RESULT MaybeObject* AllocateHeapNumber(
       double value, PretenureFlag pretenure = NOT_TENURED);
 
+  // Allocated a MutableHeapNumber from value.
+  MUST_USE_RESULT MaybeObject* AllocateMutableHeapNumber(
+      double value, PretenureFlag pretenure = NOT_TENURED);
+
   // Converts an int into either a Smi or a HeapNumber object.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.
   // Please note this does not perform a garbage collection.
   MUST_USE_RESULT inline MaybeObject* NumberFromInt32(
       int32_t value, PretenureFlag pretenure = NOT_TENURED);
 
   // Converts an int into either a Smi or a HeapNumber object.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.
@@ skipping 2002 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_