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We require offset-path to appear first, then offset-distance and
offset-rotation.

Standards discussion is continuing, this is intentionally conservative.

Spec (not yet updated):
https://drafts.fxtf.org/motion-1/#offset-shorthand

BUG=638055

Review-Url: https://codereview.chromium.org/2375343002
Cr-Commit-Position: refs/heads/master@{#423059}