Update screen.orientation to follow the latest version of the specs.

It also returns 'portrait-primary' when the frame is detached
instead of empty string (current behaviour).

BUG=162827

Review URL: https://codereview.chromium.org/207833003

git-svn-id: svn://svn.chromium.org/blink/trunk@169932 bbb929c8-8fbe-4397-9dbb-9b2b20218538