Oilpan: Change HashCountedSet in ActiveAnimation to HashMap

This is a preparation to move AnimationPlayer to oilpan's heap. Since Oilpan does not support HashCountedSet, this CL replaces HashCountedSet<AnimationPlayer*> with HashMap<AnimationPlayer*, int>.

TEST=AnimationAnimationPlayerTest::AttachedAnimationPlayers
BUG=340522

Review URL: https://codereview.chromium.org/249243002

git-svn-id: svn://svn.chromium.org/blink/trunk@172647 bbb929c8-8fbe-4397-9dbb-9b2b20218538