Adding the message writing class used for remote_security_key STDOUT communication.

remoting/host/security_key/security_key_message.h

 // 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 REMOTING_HOST_SECURITY_KEY_SECURITY_KEY_MESSAGE_H_
 #define REMOTING_HOST_SECURITY_KEY_SECURITY_KEY_MESSAGE_H_
 
 #include <cstdint>
 #include <string>
 
@@ skipping 82 matching lines @@
   // Sent by the remote_security_key if an error occurs which does not conform
   // to any existing category.  No response to this message is expected.
   // Payload length: variable.  If > 0 bytes, the bytes represent an error
   // string which is ascii encoded and does not include a null terminator.
   UNKNOWN_ERROR = 255,
 };
 
 class SecurityKeyMessage final {
  public:
   // The number of bytes used to represent the header.
-  static const int kHeaderSizeBytes = 4;
+  static const int kHeaderSizeBytes;
+
+  // The number of bytes used to represent the message type.
+  static const int kMessageTypeSizeBytes;
 
   SecurityKeyMessage();
   ~SecurityKeyMessage();
 
   // When given a header value (uint32_t), this method will return whether the
   // length is within the allowable size range.
   static bool IsValidMessageSize(uint32_t message_size);
 
   // Returns a RemoteSecurityKeyMessageType enum value corresponding to the
   // value passed in if it is valid, otherwise INVALID is returned.
@@ skipping 14 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(SecurityKeyMessage);
 };
 
 // Used to pass remote security key message data between classes.
 typedef base::Callback<void(scoped_ptr<SecurityKeyMessage> message)>
     SecurityKeyMessageCallback;
 
 }  // namespace remoting
 
 #endif  // REMOTING_HOST_SECURITY_KEY_SECURITY_KEY_MESSAGE_H_