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Adds a new Feature for integration with Unified Consent.
When this is enabled, and unified consent is on, the TTS
opt-in promo will not be needed.  We'll implement a follow-on
CL that does the conditional disabling off of this flag.

BUG=825428

Change-Id: I4becefb6bf3d1862f941bccd85c2bbc863cb99d4
Reviewed-on: https://chromium-review.googlesource.com/1148689Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577708}