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This pref will eventually be used to target onboarding features to
users who have not been using Assistant for a long time.

Bug: b:161908220
Change-Id: I336b8193182765054126abf504c786c766ff51fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314157
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791546}