[webapi][XWALK-1726] Update case code style for screenorientation

Impacted Suites: tct-screenorientation-w3c-tests
Impacted TCs num: New 0, Update 1, Delete 0
Unit test platform: Tizen(IVI), 
Tizen test result summary: Pass 0, Fail 1, Blocked 0


existence checking shall be assert_true("unlockOrientation" in screen) or https://github.com/w3c/testharness.js/blob/master/docs/api.md#assert_own_propertyobject-property_name-description