Fix unprefixed transitions when experimental features flag is off

The patch https://codereview.chromium.org/415613002 unintentionally
made unprefixed transitions depend on the animations unprefixed
flag as it made us exit out of prefixingVariantForPropertyId when
the flag was off.

BUG=404001

Review URL: https://codereview.chromium.org/478523003

git-svn-id: svn://svn.chromium.org/blink/trunk@180327 bbb929c8-8fbe-4397-9dbb-9b2b20218538