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We expect doing 3rd-party OT for FLoC, thus setting
origin_trial_allows_third_party. Also, gate the OT by a base::Feature,
as this allows us to keep the experimentation under the global usage
limit.

Bug: 1163394
Change-Id: Ibaf9554397f8d46fa68c837fe58acc66676a2401
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611953Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Yao Xiao <yaoxia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842729}