[usecase-webapi][XWALK-2270] Fix bug for MediaRenderer

- Add Precondition for the test
- Failure analysis: Cannot play video

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 #683 from yunxliu/yunxliu/misc/mediaRenderer_modify

[usecase-webapi][XWALK-2270] Fix bug for MediaRenderer