animation-direction computed value is incorrect when value is "reverse" or "alternate-reverse"

The support of these values was added in http://trac.webkit.org/changeset/107162
(18 months ago) but the computed value support was not added so it was
returning wrong values.

BUG=271573

Review URL: https://chromiumcodereview.appspot.com/22995003

git-svn-id: svn://svn.chromium.org/blink/trunk@156042 bbb929c8-8fbe-4397-9dbb-9b2b20218538