Use Chromium BUILD to approximate Blimp protocol version, and check it.

blimp/net/connection_error_observer.h

 // 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.
 
 #ifndef BLIMP_NET_CONNECTION_ERROR_OBSERVER_H_
 #define BLIMP_NET_CONNECTION_ERROR_OBSERVER_H_
 
 namespace blimp {
 
 class ConnectionErrorObserver {
  public:
   virtual ~ConnectionErrorObserver() {}
 
   // Called when a blimp connection encounters an error.
+  // Negative |error| values correspond directly to net/ error codes.
+  // Positive values indicate the EndConnection.reason code specified
+  // by the peer during a "clean" disconnection.
   virtual void OnConnectionError(int error) = 0;
 };
 
 }  // namespace blimp
 
 #endif  // BLIMP_NET_CONNECTION_ERROR_OBSERVER_H_