Make a Persistent ctor take const Persistent& instead of Persistent&.

BUG=
R=mstarzinger@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/17580010

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15278 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -509,7 +509,7 @@ template <class T> class Persistent // NOLINT
       : val_(New(isolate, *that)) { }
 
   template <class S> V8_INLINE(Persistent(Isolate* isolate,
-                                          Persistent<S>& that)) // NOLINT
+                                          const Persistent<S>& that)) // NOLINT
       : val_(New(isolate, *that)) { }
 
 #else