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Currently, we only reset the client id in the local pref when
toggling UMA enable state. We should also reset all low entropy sources
as well to be more consistent.

When a user (re)enables UMA, we immediately generate a new client id,
and start including it in UMA reports. However, we don't have to
generate a new low entropy source immediately, because we can't change
the trial assignments for the current session and we will set
`client_id_was_used_for_trial_assignment` to false on these records
so they can be excluded from most analysis. Instead, the low entropy
sources are removed from local state, which will cause new ones to be
generated and used in the next session.

Bug: 1053529
Change-Id: Ie1437e5799fea8f854e9a1bbcfbefee7652c9847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355396
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799263}