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It is important to unsubscribe WebOTPService from SMS fetcher when
it gets destructed to avoid use-after-free bugs.

This currently happens in CompleteRequest() but we only call it if there
is a callback. At the moment using the existence of a callback as a
proxy for the existence of a subscription is safe because we only
subscribe when there is a callback. But it is more robust and safer to
unsubscribe unconditionally and not rely on the existence of callback_
as an indicator for a subscription.

R=kenrb@chromium.org

BUG: 1138161
Change-Id: Ic69191fa7d895b211508b84f1b90c973f0c386a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490260Reviewed-by: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819520}