Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/arm/macro-assembler-arm.cc

Index: src/arm/macro-assembler-arm.cc
diff --git a/src/arm/macro-assembler-arm.cc b/src/arm/macro-assembler-arm.cc
index ecb8b19793625e6c67538a50822628bbdb3a96bf..4205e5605041494ea2a21d26765705ad1910e8c3 100644
--- a/src/arm/macro-assembler-arm.cc
+++ b/src/arm/macro-assembler-arm.cc
@@ -3274,14 +3274,19 @@ void MacroAssembler::AllocateHeapNumber(Register result,
                                         Register scratch2,
                                         Register heap_number_map,
                                         Label* gc_required,
-                                        TaggingMode tagging_mode) {
+                                        TaggingMode tagging_mode,
+                                        MutableMode mode) {
   // Allocate an object in the heap for the heap number and tag it as a heap
   // object.
   Allocate(HeapNumber::kSize, result, scratch1, scratch2, gc_required,
            tagging_mode == TAG_RESULT ? TAG_OBJECT : NO_ALLOCATION_FLAGS);
 
+  Heap::RootListIndex map_index = mode == MUTABLE
+      ? Heap::kMutableHeapNumberMapRootIndex
+      : Heap::kHeapNumberMapRootIndex;
+  AssertIsRoot(heap_number_map, map_index);
+
   // Store heap number map in the allocated object.
-  AssertIsRoot(heap_number_map, Heap::kHeapNumberMapRootIndex);
   if (tagging_mode == TAG_RESULT) {
     str(heap_number_map, FieldMemOperand(result, HeapObject::kMapOffset));
   } else {