Serializer: ensure unique script ids when deserializing.

test/mjsunit/deserialize-script-id.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax --cache=code
+// Test that script ids are unique and we found the correct ones.
+
+var scripts = %DebugGetLoadedScripts();
+scripts.sort(function(a, b) { return a.id - b.id; });
+var user_script_count = 0;
+scripts.reduce(function(prev, cur) {
+  assertTrue(prev === undefined || prev.id != cur.id);
+  if (cur.type == 2) user_script_count++;
+});
+
+// Found mjsunit.js and this script.
+assertEquals(2, user_script_count);