Oilpan: Make trace methods non-pure and explicitly trace them in animation/.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/263403003

git-svn-id: svn://svn.chromium.org/blink/trunk@173382 bbb929c8-8fbe-4397-9dbb-9b2b20218538