ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/ipc_message_attachment_set.h

Index: ipc/ipc_message_attachment_set.h
diff --git a/ipc/ipc_message_attachment_set.h b/ipc/ipc_message_attachment_set.h
index 5e4b195ea3f83197e55a8e2863de59ca457ed24b..893f58c2b406b48d9809ae0bc25f8822840f5ac5 100644
--- a/ipc/ipc_message_attachment_set.h
+++ b/ipc/ipc_message_attachment_set.h
@@ -60,8 +60,11 @@ class IPC_EXPORT MessageAttachmentSet
 
   // Returns a vector of all brokerable attachments.
   std::vector<const BrokerableAttachment*> PeekBrokerableAttachments() const;
-  std::vector<scoped_refptr<BrokerableAttachment>>
-  GetBrokerableAttachmentsForUpdating();
+
+  // Replaces a placeholder brokerable attachment with |attachment|, matching
+  // them by their id.
+  void ReplacePlaceholderWithAttachment(
+      const scoped_refptr<BrokerableAttachment>& attachment);
 
 #if defined(OS_POSIX)
   // This is the maximum number of descriptors per message. We need to know this