[webapi][XWALK-2533] Fix bug for mediarenderer

- Fix bug for mediarenderer

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #996 from yunxliu/yunxliu/webapi/mediarenderer

[webapi][XWALK-2533] Fix bug for mediarenderer