Add use counter analytics to CSSAnimations.

Count each specified CSS Transition every time it runs.

Count each requested CSS Animation, including those on non-animatable properties.

BUG=317604

Review URL: https://codereview.chromium.org/57683002

git-svn-id: svn://svn.chromium.org/blink/trunk@161800 bbb929c8-8fbe-4397-9dbb-9b2b20218538