Animations: Fix bug where getAnimationPlayers() crashes

ElementAnimation:getAnimationPlayers() crashes when it is called and no
animations have been previously created on the element.

BUG=396376

Review URL: https://codereview.chromium.org/423763002

git-svn-id: svn://svn.chromium.org/blink/trunk@179001 bbb929c8-8fbe-4397-9dbb-9b2b20218538