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This CL updates prefs::kSyncDemographicsBirthYearOffset to be
syncable. This change prevents a user who resets their profile
data without changing their UMA client id from generating a
new random offset, which could narrow down the true birth
year value.

Bug: 978849
Change-Id: I34d347dfdf5a35381443baa327883c72fb0aaed3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803763
Commit-Queue: Roger McFarlane <rogerm@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Tatiana Gornak <melandory@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696895}