Avoid race-warning for access to animatableAttributes
Avoid race-warning for access to animatableAttributes

Collecting debug data during a commit in CC can land us in this method
while not running in the main thread  (although it'll be blocked).
Since this is assert-only code, switch to a thread-safe initializer to
avoid triggering this warning. Hopefully this doesn't slow things down
to badly on bots.

BUG=545972

Review URL: https://codereview.chromium.org/1487813002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#363687}

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