Auto merge of #11341 - nox:raf, r=pcwalton

Use a simple Vec for the animation frame list

Adding an animation frame list is now just pushing a new pair onto the vector,
while canceling one is setting its callback to None.

This means we can't send NoAnimationCallbacksPresent anymore to the constellation
when all entries were cancelled, but I'm not sure that's very important anyway.

The good downside of this change is that when running the callbacks, if no
new one was queued during their execution, we can just swap back the original Vec
into the Document in run_the_animation_frame_callbacks, thus reusing the
original allocation.

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