Web Animations CSS: Adds the pauseForTestingOnCompositor method.

R=dstockwell@chromium.org

Review URL: https://codereview.chromium.org/77363002

git-svn-id: svn://svn.chromium.org/blink/trunk@162345 bbb929c8-8fbe-4397-9dbb-9b2b20218538