Fix usage of prefixed CSS transitions in Blink.

We're asking our users to move to unprefixed CSS properties but
we don't do it ourselves :D. Let's fix the few places where we
create CSS transitions to use the unprefixed enum value.

Review URL: https://chromiumcodereview.appspot.com/19970002

git-svn-id: svn://svn.chromium.org/blink/trunk@154664 bbb929c8-8fbe-4397-9dbb-9b2b20218538