Maintain a list of animations waiting for an async start notification

AnimationControllerPrivate currently maintains a bool indicating whether
any animations are waiting for an async start notification. This cannot
be correctly updated when animations are removed, since there's no way
to determine if the removed animation was the last animation waiting for
an async start notification. This CL changes the bool to a list.

BUG=310666,308851

Review URL: https://codereview.chromium.org/36803006

git-svn-id: svn://svn.chromium.org/blink/trunk@160405 bbb929c8-8fbe-4397-9dbb-9b2b20218538