[usecase-webapi] Improve the w3c HrtimeTizen usecase

- Change button to "Waiting" when play, and disable the button. Change back when play end.
- Put the result info "The time of video playing is" below the play button.

Impacted tests(approved): new 0, update 1, delete 0      
Unit test platform: [Tizen IVI]
Unit test result summary: pass 1, fail 0, block 0