Assigning to palmer@ and adding Enamel labels since this function is (currently) only used in the SSL error classifier.

https://codereview.chromium.org/634033002 added X509Certificate::IsSelfSigned, but it behaves differently across platforms.

It is possible to construct a certificate that is signed by the subject's key but where the subject and issuer do not match. (This would mean the subject and issuer share a key.)

The Windows implementation I think only checks the signature, but I'm not sure. (Ryan?)
https://code.google.com/p/chromium/codesearch#chromium/src/net/cert/x509_certificate_win.cc&q=X509Certificate::IsSelfSigned&sq=package:chromium&l=466&ct=rc&cd=1&dr=CSs

The iOS and Android implementations only check the signature.
https://code.google.com/p/chromium/codesearch#chromium/src/net/cert/x509_certificate_ios.cc&q=X509Certificate::IsSelfSigned&sq=package:chromium&l=462&ct=rc&cd=2&dr=CSs
https://code.google.com/p/chromium/codesearch#chromium/src/net/cert/x509_certificate_openssl.cc&q=X509Certificate::IsSelfSigned&sq=package:chromium&l=455&ct=rc&cd=5&dr=CSs

The Linux and CrOS implementations only check the signature.
https://code.google.com/p/chromium/codesearch#chromium/src/net/cert/x509_certificate_nss.cc&q=X509Certificate::IsSelfSigned&sq=package:chromium&l=284&ct=rc&cd=4&dr=CSs

The Mac implementation checks BOTH the signature and the names.
https://code.google.com/p/chromium/codesearch#chromium/src/net/cert/x509_certificate_mac.cc&q=X509Certificate::IsSelfSigned&sq=package:chromium&l=545&ct=rc&cd=3&dr=CSs

We should pick one behavior across platforms and test it. It would also be good to have a test for a self-issued but not self-signed certificate (names match but signatures don't). If we're going with the RFC 5280 definition of self-signed, I think the Mac implementation is correct and the others are missing name checks.

"Self-issued certificates are CA certificates in which the issuer and subject are the same entity.  Self-issued certificates are generated to support changes in policy or operations.  Self-signed certificates are self-issued certificates where the digital signature may be verified by the public key bound into the certificate."

Is that fine for Enamel's uses?