[webapi][XWALK-3056] Update align-self initial value for flexiblebox

- Update align-self initial value to auto following spec defined

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [IVI] crosswalk-10.39.233, t-e-c 0.120
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #1548 from xinliux/flexiblebox/XWALK-3056

[webapi][XWALK-3056] Update align-self initial value for flexiblebox