Support custom message filtering on Mojo binding objects

mojo/public/cpp/bindings/message_header_validator.h

Index: mojo/public/cpp/bindings/message_header_validator.h
diff --git a/mojo/public/cpp/bindings/message_header_validator.h b/mojo/public/cpp/bindings/message_header_validator.h
index 3bcbd0a9563c2f9c9ad94d7f4fa2ea739bcb82cb..52f02087d63d1593fda2c3b2c7b3582cb68eb43a 100644
--- a/mojo/public/cpp/bindings/message_header_validator.h
+++ b/mojo/public/cpp/bindings/message_header_validator.h
@@ -6,15 +6,13 @@
 #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.