Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 73c59df78766cebbdf2920025c2fbdfd823b076c..83ef3628dd07bade4ca4eb4e53715a2def1fd14f 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -4593,12 +4593,17 @@ void MacroAssembler::UndoAllocationInNewSpace(Register object) {
 
 void MacroAssembler::AllocateHeapNumber(Register result,
                                         Register scratch,
-                                        Label* gc_required) {
+                                        Label* gc_required,
+                                        MutableMode mode) {
   // Allocate heap number in new space.
   Allocate(HeapNumber::kSize, result, scratch, no_reg, gc_required, TAG_OBJECT);
 
+  Heap::RootListIndex map_index = mode == MUTABLE
+      ? Heap::kMutableHeapNumberMapRootIndex
+      : Heap::kHeapNumberMapRootIndex;
+
   // Set the map.
-  LoadRoot(kScratchRegister, Heap::kHeapNumberMapRootIndex);
+  LoadRoot(kScratchRegister, map_index);
   movp(FieldOperand(result, HeapObject::kMapOffset), kScratchRegister);
 }