[usecase-webapi] Fix bugs of Workers.

- 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
As to a button, I think, it shall ask for a user action. Thus, a verb is better than a noun.

See http://www.w3c-test.org/vibration/cancel-with-empty-array-manual.html