Fix Length blend() handling of Undefined lengths

Fixed transition behaviour when animating to/from 'none' keyword instead
of hitting ASSERT failure.

BUG=260760,230193

Review URL: https://chromiumcodereview.appspot.com/19518002

git-svn-id: svn://svn.chromium.org/blink/trunk@154390 bbb929c8-8fbe-4397-9dbb-9b2b20218538