Add proto for TLS error assistant, refactor proto generator code.

chrome/browser/ssl/tls_error_assistant.proto

+// 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.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package chrome_browser_ssl;
+
+message CaptivePortalCert {
+  // Sha256 hash of the certificate's public key.
+  optional string sha256_hash = 1;
+}
+
+message TLSErrorAssistantConfig {
+  optional uint32 version_id = 1;
+  repeated CaptivePortalCert captive_portal_cert = 2;
+}