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 e390052b529997ae157504e2fbbe20aea4f740f7..f1ae4487d7464ddfdb8796a8828a9b04766d9f4f 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -316,6 +316,9 @@ class Factory {
                                    PretenureFlag pretenure = NOT_TENURED);
   Handle<HeapNumber> NewHeapNumber(double value,
                                    PretenureFlag pretenure = NOT_TENURED);
+  Handle<HeapNumber> NewMutableHeapNumber(
+      double value,
+      PretenureFlag pretenure = NOT_TENURED);
 
 
   // These objects are used by the api to create env-independent data