This is preparation for sharing this code with layout_2020 and
implementing selective off-the-main-thread animations.
We still look for nodes not in the flow tree in the layout thread.
This is a small step toward fixing #19242. The main idea is that the
clock for animations should advance as the event loop ticks. We
accomplish this by moving the clock from layout and naming it the
"animation timeline" which is the spec language. This should fix
flakiness with animations and transitions tests where a reflow could
move animations forward while script was running.
This change also starts to break out transition and animation events
into their own data structure, because it's quite likely that the next
step in fixing #19242 is to no longer send these events through a
channel.
