[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


s/Screen_onorientationchange_attribute_tests/Screen_onorientationchange/

Or you can put together all the attributes/methods existence and type checking into one test page :)

The 1st test case shall check window.screen exists.