[usecase-webapi] Improve Input usecase.

- Remove three buttons, and integrate to 'index.html'.
- Fail Reason: xwalk-1456.

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 #712 from minxhuang/hm/misc/usecase-w3c/Input

[usecase-webapi] Improve Input usecase.