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When a layer with an animation is removed from the tree but later
re-added (this can happen, for example, when an iframe containing
an animation is set to display:none and later set back to
display:block), the animation should continue. Currently, the
animation is prevented from getting re-pushed to the compositor
thread since it already has a start time. This logic used to be
needed to prevent incorrectly re-pushing an animation that had
already been deleted  on the compositor thread, but is no longer
needed for that purpose since we now always delete animations on
the main thread before deleting them on the compositor thread.
This CL removes this logic.

BUG=409649

Review URL: https://codereview.chromium.org/1148333002

Cr-Commit-Position: refs/heads/master@{#330938}