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Summary of changes:
  * Sync now handles all the logic of retrieving demographics, which includes deciding when and when not to provide demographics.
  * Added computation of the user age using the now time.
  * Added a check to not report demographics when user is not old enough.
  * Added age fuzzing.
  * Added more unittests for SyncPrefs::GetUserDemographics().
  * Added demographic checks to make sure that provided demographics have low entropy to protect privacy.
  * Added more explanations about privacy protection.

Bug: 983303
Change-Id: I4ef12059b0ad71a6dc656de0d59e8e9369ba6111
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707109
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680396}