[webapi][XWALK-2560] Fix bug for mediaserver

- Fix bug for mediaserver
- Failure analysis: https://crosswalk-project.org/jira/browse/XWALK-1701

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 0, fail 1, block 0
I have moved these checkpoints to demo-express. https://github.com/crosswalk-project/demo-express/pull/17. When this pr is merged, I will remove these checkpoints on https://github.com/crosswalk-project/crosswalk-test-suite/pull/960.