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The CertIssuerSource interface will provide an abstraction for the path builder to
lookup issuers without needing to know the details of how.

CertIssuerSourceStatic is a simple implementation useful for providing a fixed set,
for example the certs received during the TLS handshake.

BUG=410574

Review-Url: https://codereview.chromium.org/2030693002
Cr-Commit-Position: refs/heads/master@{#397899}