Use AnimationData::initialAnimationTimingFunction() in CSSAnimations::timingFromAnimationData()

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23451019

git-svn-id: svn://svn.chromium.org/blink/trunk@157447 bbb929c8-8fbe-4397-9dbb-9b2b20218538