[Blimp Net] Add EngineAuthHandler.

blimp/common/proto/protocol_control.proto

+// Copyright 2015 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 blimp;
+
+// Contains Client->Engine information to establish a authenticated connection
+message StartConnectionMessage {
+  optional string client_token = 1;
+  optional int32 protocol_version = 2;
+  // TODO(haibinlu): add checkpoints.
+}
+
+message ProtocolControlMessage {
+  enum Type {
+    UNKNOWN = 0;
+
+    // Client => Server types.
+    START_CONNECTION = 1;
+  }
+
+  optional Type type = 1;
+
+  optional StartConnectionMessage start_connection = 1000;
+}