Sync VTTCue.align/vertical with spec

http://dev.w3.org/html5/webvtt/#vttcue-interface

The tests have already been fixed in web-platform-tests:
https://github.com/w3c/web-platform-tests/commit/5e4f41f421cc87e8bdd154043e20531cc79ec76d

BUG= 367555

Review URL: https://codereview.chromium.org/254833003

git-svn-id: svn://svn.chromium.org/blink/trunk@172854 bbb929c8-8fbe-4397-9dbb-9b2b20218538