Auto merge of #9401 - glennw:raf-timing, r=jdm

Fixes additional calls to rAF.

Often, a rAF callback will request another rAF from the callback itself.

Previously, the constellation would quickly receive two messages saying
that there were no animations, and then there are animations again in the
situation above. This would make the compositor tick the new animation straight
away, causing strange fluctuations and timings in rAF callbacks.

Instead, only send the NoAnimationCallbacks message if the animation
callback queue is still empty after invoking the callbacks.

This fixes rAF timing, which now runs at the correct (vsync) framerate.

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