Use mutable heapnumbers to store doubles in fields.

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index ca6ad41be68fb215ed7d160e646869b10bed6b0b..5e8970835b3d7dfc425496df4bc69bddaf42d804 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -267,6 +267,9 @@ class Factory {
   Handle<Object> NewNumberFromUint(uint32_t value,
                                   PretenureFlag pretenure = NOT_TENURED);
 
+  Handle<HeapNumber> NewHeapNumber(double value,
+                                   PretenureFlag pretenure = NOT_TENURED);
+
   // These objects are used by the api to create env-independent data
   // structures in the heap.
   Handle<JSObject> NewNeanderObject();