Run 2-keyframe animations with non-linear timing on the compositor.

The animation has to have a linear timing function on the first keyframe.

BUG=

Review URL: https://codereview.chromium.org/472853003

git-svn-id: svn://svn.chromium.org/blink/trunk@180842 bbb929c8-8fbe-4397-9dbb-9b2b20218538