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 0f586e9284e5554a97911982c4c441dc2e44aaec..204c2ba1ca0f742bfbb6477ad98d39088ac7848f 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)                                                    \
@@ -246,6 +247,7 @@ 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)                             \
@@ -1119,7 +1121,9 @@ class Heap {
 
   // Allocated a HeapNumber from value.
   MUST_USE_RESULT MaybeObject* AllocateHeapNumber(
-      double value, PretenureFlag pretenure = NOT_TENURED);
+      double value,
+      MutableMode mode = IMMUTABLE,
+      PretenureFlag pretenure = NOT_TENURED);
 
   // Converts an int into either a Smi or a HeapNumber object.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation