ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/ipc_message.h

 // Copyright (c) 2012 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 IPC_IPC_MESSAGE_H_
 #define IPC_IPC_MESSAGE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 176 matching lines @@
     // ids of the brokerable attachments. The caller of FindNext() is
     // responsible for adding the attachments to the message.
     std::vector<BrokerableAttachment::AttachmentId> attachment_ids;
   };
 
   // |info| is an output parameter and must not be nullptr.
   static void FindNext(const char* range_start,
                        const char* range_end,
                        NextMessageInfo* info);
 
+  // Adds a placeholder brokerable attachment that must be replaced before the
+  // message can be dispatched.
+  bool AddPlaceholderBrokerableAttachmentWithId(
+      BrokerableAttachment::AttachmentId id);
+
   // WriteAttachment appends |attachment| to the end of the set. It returns
   // false iff the set is full.
   bool WriteAttachment(scoped_refptr<MessageAttachment> attachment);
   // ReadAttachment parses an attachment given the parsing state |iter| and
   // writes it to |*attachment|. It returns true on success.
   bool ReadAttachment(base::PickleIterator* iter,
                       scoped_refptr<MessageAttachment>* attachment) const;
   // Returns true if there are any attachment in this message.
   bool HasAttachments() const;
   // Returns true if there are any MojoHandleAttachments in this message.
@@ skipping 94 matching lines @@
   MSG_ROUTING_NONE = -2,
 
   // indicates a general message not sent to a particular tab.
   MSG_ROUTING_CONTROL = INT32_MAX,
 };
 
 #define IPC_REPLY_ID 0xFFFFFFF0  // Special message id for replies
 #define IPC_LOGGING_ID 0xFFFFFFF1  // Special message id for logging
 
 #endif  // IPC_IPC_MESSAGE_H_