[webapi] Fix screen orientation test cases according to spec.

1, We need to use a valid functionn to wrap the "fake" function callback (e.g. unreached_func() in this case).
2, According to screen orientation w3c spec http://www.w3.org/TR/screen-orientation/#orientationlocktype-enum, the "OrientationLockType" cannot be angle value such as "0", "90", "180", "270"
3, The screen.orientation.lock() cannot be invoked in parallel. That might cause the JavaScript Promise object's resolved callback not be invoked. Instead, we should call them in sequence.

BUG=XWALK-2513
Merge pull request #1205 from yejingfu/bug-fix

[webapi] Fix screen orientation test cases according to spec.