Screen Orientation: fire event on Window instead of Screen.

This is following spec discussions in public-webapps.

BUG=162827

Review URL: https://codereview.chromium.org/241203003

git-svn-id: svn://svn.chromium.org/blink/trunk@172152 bbb929c8-8fbe-4397-9dbb-9b2b20218538