Move more Mojo bindings helpers out of internal namespace

mojo/public/cpp/bindings/lib/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_LIB_MESSAGE_HEADER_VALIDATOR_H_
-#define MOJO_PUBLIC_CPP_BINDINGS_LIB_MESSAGE_HEADER_VALIDATOR_H_
-
-#include "mojo/public/cpp/bindings/message.h"
-#include "mojo/public/cpp/bindings/message_filter.h"
-
-namespace mojo {
-namespace internal {
-
-class MessageHeaderValidator : public MessageFilter {
- public:
-  explicit MessageHeaderValidator(MessageReceiver* sink = nullptr);
-  MessageHeaderValidator(const std::string& description,
-                         MessageReceiver* sink = nullptr);
-
-  // 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 internal
-}  // namespace mojo
-
-#endif  // MOJO_PUBLIC_CPP_BINDINGS_LIB_MESSAGE_HEADER_VALIDATOR_H_