Move animation compositing trigger to be a styleDeterminedReason

Now that the animation trigger depends only on style information we can save 4%
of the time in the calculator's compositing update by recomputing the trigger
only when style changes.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/227673009

git-svn-id: svn://svn.chromium.org/blink/trunk@171027 bbb929c8-8fbe-4397-9dbb-9b2b20218538