Allow enabling 'ScreenOrientation' feature outside Blink.

Upstream link: https://codereview.chromium.org/336513003/

SSIA. This patch introduces the 'enableScreenOrientation()' setter
to the WebRuntimeFeatures class, so that 'ScreenOrientation' feature
can be enabled outside Blink.
Merge pull request #15 from pozdnyakov/enableScreenOrientation

Allow enabling 'ScreenOrientation' feature outside Blink.