Adding the message writing class used for remote_security_key STDOUT communication.

remoting/host/security_key/security_key_message.cc

Index: remoting/host/security_key/security_key_message.cc
diff --git a/remoting/host/security_key/security_key_message.cc b/remoting/host/security_key/security_key_message.cc
index 9c02821eb81091f9efda6d09f3643454eb174109..34de69a98241482dd0b8663097cc23de7332cc6a 100644
--- a/remoting/host/security_key/security_key_message.cc
+++ b/remoting/host/security_key/security_key_message.cc
@@ -11,12 +11,14 @@ namespace {
 // Limit remote security key messages to 256KB.
 const uint32_t kMaxSecurityKeyMessageByteCount = 256 * 1024;
 
-const int kRemoteSecurityKeyMessageControlCodeByteCount = 1;
-
 }  // namespace
 
 namespace remoting {
 
+const int SecurityKeyMessage::kHeaderSizeBytes = 4;
+
+const int SecurityKeyMessage::kMessageTypeSizeBytes = 1;
+
 SecurityKeyMessage::SecurityKeyMessage() {}
 
 SecurityKeyMessage::~SecurityKeyMessage() {}
@@ -75,7 +77,7 @@ bool SecurityKeyMessage::ParseMessage(const std::string& message_data) {
   }
 
   payload_.clear();
-  if (message_data.size() > kRemoteSecurityKeyMessageControlCodeByteCount) {
+  if (message_data.size() > kMessageTypeSizeBytes) {
     payload_ = message_data.substr(1);
   }