Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/arm/stub-cache-arm.cc

Index: src/arm/stub-cache-arm.cc
diff --git a/src/arm/stub-cache-arm.cc b/src/arm/stub-cache-arm.cc
index 1c772eec81152d77ade1e95a7ebe1135d93210fb..c30fa1f881a22134fa30787e0bab8aba9257cce7 100644
--- a/src/arm/stub-cache-arm.cc
+++ b/src/arm/stub-cache-arm.cc
@@ -426,8 +426,9 @@ void StoreStubCompiler::GenerateStoreTransition(MacroAssembler* masm,
     }
   } else if (representation.IsDouble()) {
     Label do_store, heap_number;
-    __ LoadRoot(scratch3, Heap::kHeapNumberMapRootIndex);
-    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, scratch3, slow);
+    __ LoadRoot(scratch3, Heap::kMutableHeapNumberMapRootIndex);
+    __ AllocateHeapNumber(storage_reg, scratch1, scratch2, scratch3, slow,
+                          TAG_RESULT, MUTABLE);
 
     __ JumpIfNotSmi(value_reg, &heap_number);
     __ SmiUntag(scratch1, value_reg);