Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 00f20ff8ba4c4a3435f136abbca2aee8a07930f8..30f4f8ddae65207616318264f2ad3ebecb61ca12 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -315,9 +315,9 @@ class Factory {
   inline Handle<Object> NewNumberFromSize(size_t value,
                                    PretenureFlag pretenure = NOT_TENURED);
   Handle<HeapNumber> NewHeapNumber(double value,
+                                   MutableMode mode = IMMUTABLE,
                                    PretenureFlag pretenure = NOT_TENURED);
 
-
   // These objects are used by the api to create env-independent data
   // structures in the heap.
   Handle<JSObject> NewNeanderObject();