[usecase-webapi] Improve WebMessaging usecase

- Fix the mistake of purpose in WebMessaging.

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 #708 from minxhuang/hm/misc/usecase-w3c/WebMessaging

[usecase-webapi] Improve WebMessaging usecase