[usecase-webapi] Improve Flexible w3c usecase

- Draw the flex box one by one in the first page
- Give a meaningful description of pass/fail criteria whether in sub test title, or case details.

Impacted tests(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 #578 from zhuyongyong/yongzhu/usecase-webapi/flexibleBox

[usecase-webapi] Improve Flexible w3c usecase