[usecase-webapi] Make usecase of Workers better.

- Modify the button name 'Start' to 'Calculation'.

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 #523 from minxhuang/hm/misc/usecase-w3c/Workers-hotfix

[usecase-webapi] Improve Workers usecase.