Use keyframe easing for CSS transitions instead of animation easing

In crrev.com/265453005 I changed the easing for CSS transitions to be on
the Animation level instead of the Keyframe level, but we only support
compositor animations with linear animation timing function.
This patch moves the easing back to the keyframe level so transitions
can be accelerated again.

BUG=372901

Review URL: https://codereview.chromium.org/282103007

git-svn-id: svn://svn.chromium.org/blink/trunk@174224 bbb929c8-8fbe-4397-9dbb-9b2b20218538