Web Animations: Add a name to AnimationNode and trace running players

For CSS Animations the name is prefilled with the animation-name. For
Transitions the name is the transitioning property.

BUG=351446

Review URL: https://codereview.chromium.org/535843002

git-svn-id: svn://svn.chromium.org/blink/trunk@181724 bbb929c8-8fbe-4397-9dbb-9b2b20218538