[webapi][XWALK-2313] Fixed timeout by adding time for screenorientation

- QA test on Samsung Galaxy tab, sometime can pass, so adding onload_delay time
- Failure reason: We test on IVI, onorientationchange event can't invoked.

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: Tizen IVI
Tizen test result summary: Pass 0, Fail 1, Blocked 0