Auto merge of #26363 - mrobinson:animation-split-update-style, r=emilio

Split animation cancellation from update_style_for_animation

`update_style_for_animation` previously handled both canceling defunct
animations and also updating style to reflect current animation state.
This change splits those two concerns because we want to start handling
replaced or canceled animations and finished animations in two different
places.

This is a refactor, so ideally it shouldn't change any behavior.

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