This was there so that crash client id could be cleared.
However, this was being called out of MetricsService::Stop(),
so it needed a special provision to not clear it when shutting
down, for example.

This change instead removes all of that plumbing and makes the
id clearing happen in the same place we clear the UMA client id,
when the user actually opts-out (as opposed to just metrics service
stopping). Unifies the prefs manipulation that happens when the
user changes the setting into a helper function.

As a follow up change after this lands, we should also make that
function be called on iOS.

BUG=635669
TBR=sgurun@chromium.org,nyquist@chromium.org,noyau@chromium.org

Review-Url: https://codereview.chromium.org/2248793002
Cr-Commit-Position: refs/heads/master@{#412568}