[web-animations] Refine definition of finished event dispatch

--HG--
extra : rebase_source : 9decb086d6b33197678de3841e705236e223afaf
[web-animations] Clean up change log

The change log is for changes since the last publication. The effective point
offset definition wasn't in the last publication so we don't record incremental
changes to it--only the fact that it was added.