ipc: Make a new class PlaceholderBrokerableAttachment.

ipc/ipc_message.cc

Index: ipc/ipc_message.cc
diff --git a/ipc/ipc_message.cc b/ipc/ipc_message.cc
index cba89545f875ecd758a88331fee42f6dccc4d6db..99868a39ef71dbc90c9d30cabd94e79cc41e3daf 100644
--- a/ipc/ipc_message.cc
+++ b/ipc/ipc_message.cc
@@ -11,6 +11,7 @@
 #include "build/build_config.h"
 #include "ipc/ipc_message_attachment.h"
 #include "ipc/ipc_message_attachment_set.h"
+#include "ipc/placeholder_brokerable_attachment.h"
 
 #if defined(OS_POSIX)
 #include "base/file_descriptor_posix.h"
@@ -180,6 +181,13 @@ void Message::FindNext(const char* range_start,
   info->message_found = true;
 }
 
+bool Message::AddPlaceholderBrokerableAttachmentWithId(
+    BrokerableAttachment::AttachmentId id) {
+  scoped_refptr<PlaceholderBrokerableAttachment> attachment(
+      new PlaceholderBrokerableAttachment(id));
+  return attachment_set()->AddAttachment(attachment);
+}
+
 bool Message::WriteAttachment(scoped_refptr<MessageAttachment> attachment) {
   // We write the index of the descriptor so that we don't have to
   // keep the current descriptor as extra decoding state when deserialising.