Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index f6f65252214157abd920ea58f4e3cc89ad323893..68150ca2347b61b6fcc0c58dac4e667bf0d64306 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -1027,6 +1027,15 @@ Handle<HeapNumber> Factory::NewHeapNumber(double value,
 }
 
 
+Handle<HeapNumber> Factory::NewMutableHeapNumber(double value,
+                                                 PretenureFlag pretenure) {
+  CALL_HEAP_FUNCTION(
+      isolate(),
+      isolate()->heap()->AllocateMutableHeapNumber(value, pretenure),
+      HeapNumber);
+}
+
+
 Handle<JSObject> Factory::NewNeanderObject() {
   CALL_HEAP_FUNCTION(
       isolate(),