crypto/tls: export the verified chains.

The verified chains are the chains that were actually verified.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4462046

common.go

@@ -100,6 +100,8 @@ type ConnectionState struct {
 
 	// the certificate chain that was presented by the other side
 	PeerCertificates []*x509.Certificate
+	// the verified certificate chains built from PeerCertificates.
+	VerifiedChains [][]*x509.Certificate
 }
 
 // A Config structure is used to configure a TLS client or server. After one

conn.go

@@ -768,6 +768,7 @@ func (c *Conn) ConnectionState() ConnectionState {
 		state.NegotiatedProtocolIsMutual = !c.clientProtocolFallback
 		state.CipherSuite = c.cipherSuite
 		state.PeerCertificates = c.peerCertificates
+		state.VerifiedChains = c.verifiedChains
 	}
 
 	return state