Make sure unprefixed animation shorthands work.

When we parse the animation shorthand we need to add the
unprefixed and prefixed longhands otherwise if we add only
the unprefixed it won't be resolved.

BUG=341003

Review URL: https://codereview.chromium.org/183483002

git-svn-id: svn://svn.chromium.org/blink/trunk@168093 bbb929c8-8fbe-4397-9dbb-9b2b20218538