Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index fde95ce1f7984ea1f6fe683d4ab982eaa445d991..fc5fdc397edcc814b0876afcdf835488d3277175 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -65,6 +65,7 @@ namespace internal {
   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)                                                    \
@@ -236,6 +237,7 @@ namespace internal {
   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)                             \
@@ -1111,6 +1113,10 @@ class Heap {
   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.