CSS Animations: Add a test of the timing model

This exposes numerous problems in the existing implementation
that should be resolved by moving to the Web Animations engine.

Some issues dealing with 0 iteration / 0 duration animations in
Web Animations are also exposed.

Review URL: https://codereview.chromium.org/25387002

git-svn-id: svn://svn.chromium.org/blink/trunk@158812 bbb929c8-8fbe-4397-9dbb-9b2b20218538