Screen Orientation API: screen.orientation & orientationchange event

Screen Orientation API: add implemention for screen.orientation attribute, keep
it updated via the ScreenOrientationController and notify client of orientation
changes via the 'orientationchange' event.

The layout test infrastructure is still missing to test this. It will be added
in a follow-up patch.

R=abarth
BUG=162827

Review URL: https://codereview.chromium.org/169403006

git-svn-id: svn://svn.chromium.org/blink/trunk@167477 bbb929c8-8fbe-4397-9dbb-9b2b20218538