CSS Animations: Use style.setProperty in interpolation tests

Blink allows style['dashed-name'] but Gecko only style['lowerCamel'],
setProperty allows dashed names in both engines.

NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/23570004

git-svn-id: svn://svn.chromium.org/blink/trunk@157104 bbb929c8-8fbe-4397-9dbb-9b2b20218538