Support custom message filtering on Mojo binding objects

mojo/public/cpp/bindings/message_header_validator.h

 // Copyright 2014 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 MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_HEADER_VALIDATOR_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_HEADER_VALIDATOR_H_
 
 #include "mojo/public/cpp/bindings/message.h"
-#include "mojo/public/cpp/bindings/message_filter.h"
 
 namespace mojo {
 
-class MessageHeaderValidator : public MessageFilter {
+class MessageHeaderValidator : public MessageReceiver {
  public:
-  explicit MessageHeaderValidator(MessageReceiver* sink = nullptr);
-  MessageHeaderValidator(const std::string& description,
-                         MessageReceiver* sink = nullptr);
+  MessageHeaderValidator();
+  explicit MessageHeaderValidator(const std::string& description);
 
   // Sets the description associated with this validator. Used for reporting
   // more detailed validation errors.
   void SetDescription(const std::string& description);
 
   bool Accept(Message* message) override;
 
  private:
   std::string description_;
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_BINDINGS_MESSAGE_HEADER_VALIDATOR_H_