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CertVerifier today guarantees that if the associated
CertVerifier::Request is deleted, the caller-supplied
callback will not be invoked. However, it does not provide
any guarantees as to when the callback itself will be
freed.

This strengthens the guarantee, such that when a ::Request
is deleted, it will also Reset() the callback. Similarly,
it provides a strong guarantee that if the CertVerifier is
deleted, the associated Callbacks will be Reset. Put
differently, if either (CertVerifier is deleted ||
Request is deleted), then the Callback will be Reset. This
makes it easier for callers to be confident of an
"eventually consistent" approach.

Prior to this change, when the Callback was destroyed was
a bit indeterminate. In some cases, when the underlying
CertVerifier went away, the Callback would be destroyed. In
other cases, it was not until the ::Request was deleted by
the caller. And in one, it was not until both the ::Request
was deleted and the underlying CertVerifier had been
deleted/the asynchronous job completed before the callback
would be destroyed.

Bug: none
Change-Id: I527fa9f3700016269d4236bae03c13510ab69b7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862116Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707472}