[usecase-webapi] Improve w3c usecase FlexibleBox

- Add black top bar for FlexibleBox page

Impacted tests(designed|approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI][Android]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #709 from zhuyongyong/yongzhu/usecase-webapi/flexibleBox-improve

[usecase-webapi] Improve w3c usecase FlexibleBox