[webapi][XWALK-2355] Rewrite the test case checking Navigator.system attribute

- Separate navigator.system and xwalk.experimental.system
  as Crosswalk implements the latter
- Use assert_readonly to check readonly attribute
- Update copyright year to 2014 as it was created in this year originally
Merge pull request #734 from zqzhang/zqzhang/XWALK-2355

[webapi][XWALK-2355] Rewrite the test case checking Navigator.system attribute