Web Animations: Fix a bug where a null directed time was being passed to the timing function

Spec: See step 1 http://www.w3.org/TR/2013/WD-web-animations-20130625/#calculating-the-transformed-time
BUG=240653

Review URL: https://chromiumcodereview.appspot.com/19792010

git-svn-id: svn://svn.chromium.org/blink/trunk@154629 bbb929c8-8fbe-4397-9dbb-9b2b20218538