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FamilyUserMetricsProvider currently causes a crash in guest mode. This
CL fixes the crash and categorizes guest users into the other bucket.

This CL also restructures the FamilyUserMetricsProvider to an
observer-based approach that improves stability and avoids unnecessary
work by caching results.

Bug: 1137352,1103077
Change-Id: Ie421861970f6b26def1cf7390bd89480081bcab5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476897Reviewed-by: Aga Wronska <agawronska@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Toby Huang <tobyhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819118}