This removes the global FetchClientCertPrivateKey function, which needed to be able
to find the private key given just an X509Certificate object. Now, ClientCertStore
returns ClientCertIdentity objects, which contain a certificate and have a method to
asynchronously retrieve the matching SSLPrivateKey object.

Also re-enable SSLClientCertificateSelectorCocoaTest.Basic.

BUG=394131,313243,222296

Review-Url: https://codereview.chromium.org/2898573002
Cr-Commit-Position: refs/heads/master@{#480556}