Modify CSSAnimationData::timingFunction() to return a raw pointer

Currently this method returns a PassRefPtr, but most call sites require a raw
pointer. This change reduces ref-count churn in those cases.

No functional change.

R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/24054004

git-svn-id: svn://svn.chromium.org/blink/trunk@157557 bbb929c8-8fbe-4397-9dbb-9b2b20218538