Mac: Look for issuer in a certificate chain instead of subject

If the CA does not send us intermediate certificates, we may not be able to
build a chain up to the relevant issuer. Instead, search the chain for
certificates with just the correct issuer.

R=wtc
BUG=50980
TEST=X509CertificateTest.IsIssuedBy

Review URL: http://codereview.chromium.org/3052035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@54780 0039d316-1c4b-4281-b951-d872f2087c98