SecTrustSetKeychains does not work on Mac OS 10.12 (Sierra)

SecTrustSetKeychains is used for the keychain reordering part of the loop-and-chop workaround in CertVerifyProcMac (see https://crrev.com/af868e7814).

Affects tests CertVerifyProcTest.MacCRLIntermediate (fails) and CertVerifyProcTest.MacKeychainReordering (passes, but not for the right reason). Also affects a WIP CL I have for client cert testing with intermediate certs (https://codereview.chromium.org/2411023002/).