Add a new GetTlsUniqueChannelBinding method to SSLSocket, and implement nss version.

net/socket/ssl_socket.h

Index: net/socket/ssl_socket.h
diff --git a/net/socket/ssl_socket.h b/net/socket/ssl_socket.h
index bd063a425af7d10bdd77fb3bdf9c959f63e5980e..86c03db0f595e19f58fcff524dc766681f48dfcf 100644
--- a/net/socket/ssl_socket.h
+++ b/net/socket/ssl_socket.h
@@ -27,6 +27,9 @@ public:
                                    const base::StringPiece& context,
                                    unsigned char* out,
                                    unsigned int outlen) = 0;
+
+  // Stores the the tls-unique channel binding (see RFC 5929) in |*out|.
+  virtual int GetTLSUniqueChannelBinding(std::string* out) = 0;
 };
 
 }  // namespace net