This patch switches on the new, "actually start the player in the next refresh
driver tick" behavior. It updates PendingPlayerTracker, adding
a StartPendingPlayersOnNextTick method which calls the appropriate method on
AnimationPlayer. The existing StartPendingPlayers is renamed to
StartPendingPlayersNow and is used for testing only.
Furthermore, since we now expect AnimationPlayer::StartOnNextTick to be
functional, AnimationPlayer::DoPlay is updated to use it when there is no
document available. This should make playing an animation player always
asynchronous, that is, always transition to the pending state temporarily
(unless we are already playing).
We would like to trigger animations from the point when the first frame of the
animation is painted. However, some animations will never trigger a paint (e.g.
animations with an empty keyframes rule). These animations should start start
however. To ensure this, whenever an animation is newly pending we schedule
a paint.
This patch adds a reference from PendingPlayerTracker back to the document
object that owns it. This is used in the next patch in this series to find the
document's root frame for scheduling a paint.
This patch adds a hashtable to nsDocument that stores all the animation players
that are currently waiting to start. In the future it may also be used to store
players that are waiting to pause which is why the methods are called
AddPlayPending/RemovePlayPending instead of just AddPlayer/RemovePlayer.
