[usecase-webapi] Fix the bug that the 'Pass' button is disabled when click the 'Cancle CSS FullScreen' button.

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 #687 from minxhuang/hm/misc/usecase-w3c/FullScreen

[usecase-webapi] Fix the bug that the 'Pass' button is disabled when click the 'Cancle CSS FullScreen' button.