Use HeapHashCountedSet for AnimationPlayerCountedSet

In r172647, I changed the AnimationPlayerCountedSet from a HashCountedSet to a HashMap, because at that time Oilpan didn't support a HashCountedSet. Now that Oilpan supports a HashCountedSet, this CL changes it back to a HashCountedSet again.

BUG=340522

Review URL: https://codereview.chromium.org/425903003

git-svn-id: svn://svn.chromium.org/blink/trunk@179097 bbb929c8-8fbe-4397-9dbb-9b2b20218538