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offset-position and offset-anchor are both animatable as
<position>.
https://drafts.fxtf.org/motion-1/#offset-position-property
https://drafts.fxtf.org/motion-1/#offset-anchor-property

BUG=638055

Review-Url: https://codereview.chromium.org/2364503002
Cr-Commit-Position: refs/heads/master@{#420581}