[webapi][XWALK-2572] Fix bug for mediaserver

- Fix bug for mediaserver
- Failure analysis: https://crosswalk-project.org/jira/browse/XWALK-1699

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 0, fail 1, block 0
Merge pull request #961 from yunxliu/yunxliu/webapi/mediaserver_xwalk2572

[webapi][XWALK-2572] Fix bug for mediaserver