Add a test for explicitly setting the transition-property property to 'initial'

In particular, test that this works for both prefixed and unprefixed versions.

R=dstockwell

Review URL: https://codereview.chromium.org/81783003

git-svn-id: svn://svn.chromium.org/blink/trunk@162670 bbb929c8-8fbe-4397-9dbb-9b2b20218538