Correct const accessors in base/values.(h|cc), Part II (ListValue)

ipc/ipc_message_utils.cc

Index: ipc/ipc_message_utils.cc
diff --git a/ipc/ipc_message_utils.cc b/ipc/ipc_message_utils.cc
index da9b48b5c460d3c404fc22c74147c85f59a013f8..3ce9be53b3672cf1b9f1642bb39b64d326140aaa 100644
--- a/ipc/ipc_message_utils.cc
+++ b/ipc/ipc_message_utils.cc
@@ -117,7 +117,7 @@ void WriteValue(Message* m, const Value* value, int recursion) {
       const ListValue* list = static_cast<const ListValue*>(value);
       WriteParam(m, static_cast<int>(list->GetSize()));
       for (size_t i = 0; i < list->GetSize(); ++i) {
-        Value* subval;
+        const Value* subval;
         if (list->Get(i, &subval)) {
           WriteValue(m, subval, recursion + 1);
         } else {