[web-animations] Fix motion path pacing algorithm

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extra : rebase_source : f18ba650e7cfe2471c17a5b3d30f7fa68e1bff62
[web-animations] Make the composite operation a property of all animation effects

--HG--
extra : rebase_source : 106392ce19db183d54fe3a2bd0e7ae96e7b55822