Add optional context for certificate errors.

net/cert/internal/cert_error_id.h

Index: net/cert/internal/cert_error_id.h
diff --git a/net/cert/internal/cert_error_id.h b/net/cert/internal/cert_error_id.h
new file mode 100644
index 0000000000000000000000000000000000000000..062ba4069f73414c9ba44141be8ef758560d9489
--- /dev/null
+++ b/net/cert/internal/cert_error_id.h
@@ -0,0 +1,37 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NET_CERT_INTERNAL_CERT_ERROR_ID_H_
+#define NET_CERT_INTERNAL_CERT_ERROR_ID_H_
+
+#include "net/base/net_export.h"
+
+namespace net {
+
+// Each "class" of certificate error/warning has its own unique ID. This is
+// essentially like an error code, however the value is not stable. Under the
+// hood these IDs are pointers and use the process's address space to ensure
+// uniqueness.
+//
+// Equality of CertErrorId can be done using the == operator.
+//
+// To define new error IDs use the macro DEFINE_CERT_ERROR_ID().
+using CertErrorId = const void*;
+
+// DEFINE_CERT_ERROR_ID() creates a CertErrorId given a non-null C-string
+// literal. The string should be a textual name for the error which will appear
+// when pretty-printing errors for debugging. It should be ASCII.
+//
+// TODO(crbug.com/634443): Implement this -- add magic to ensure that storage
+//                         of identical strings isn't pool.
+#define DEFINE_CERT_ERROR_ID(name, c_str_literal) \
+  CertErrorId name = c_str_literal
+
+// Returns a debug string for a CertErrorId. In practice this returns the
+// string literal given to DEFINE_CERT_ERROR_ID(), which is human-readable.
+NET_EXPORT const char* CertErrorIdToDebugString(CertErrorId id);
+
+}  // namespace net
+
+#endif  // NET_CERT_INTERNAL_CERT_ERROR_ID_H_