[screen-orientation] add ability to lock the screen to its natural orientation.

See spec change: https://github.com/w3c/screen-orientation/pull/18

BUG=162827

Review URL: https://codereview.chromium.org/343173004

git-svn-id: svn://svn.chromium.org/blink/trunk@176778 bbb929c8-8fbe-4397-9dbb-9b2b20218538