Web Animations: In ActiveAnimations, track players with animations targeting the corresponding element

This will support implementing compositor animations and later
Element.getCurrentPlayers.

This subtly changes the behavior of Element::activeAnimations to return an
ActiveAnimations instance once it has been initialized rather than only when
it contains active animations.

Review URL: https://codereview.chromium.org/58893008

git-svn-id: svn://svn.chromium.org/blink/trunk@161987 bbb929c8-8fbe-4397-9dbb-9b2b20218538