ActiveAnimations::isEmpty() should check the existence of AnimationPlayers

This CL fixes a failure of web-animations-api/w3c/getAnimationPlayers.html.

In both oilpan and non-oilpan builds, ActiveAnimations::isEmpty() should check m_players.isEmpty(), which returns true when there is any AnimationPlayer attached to the Element that has the ActiveAnimations.

BUG=398556

Review URL: https://codereview.chromium.org/431443002

git-svn-id: svn://svn.chromium.org/blink/trunk@179236 bbb929c8-8fbe-4397-9dbb-9b2b20218538