[usecase-webapi] Improve Viewport usecase.

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 #584 from minxhuang/hm/misc/usecase-w3c/Viewport-hotfix

[usecase-webapi] Improve Viewport usecase.