net/cert: Use ListCertsCallback typedef for ListCerts() parameter.
net/cert: Use ListCertsCallback typedef for ListCerts() parameter.

This way we don't duplicate the typedef of NSSCertDatabase.

BUG=None
R=eroman@chromium.org

Review URL: https://codereview.chromium.org/1414203006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357685}

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