IPC: Add attachment brokering support to the message header.

ipc/ipc_channel_reader.h

Index: ipc/ipc_channel_reader.h
diff --git a/ipc/ipc_channel_reader.h b/ipc/ipc_channel_reader.h
index 6f4b870df0da00f80fe57ab79e639e80a0b85bd3..97f67479df3785e1e2d85048c972c1456f4b757f 100644
--- a/ipc/ipc_channel_reader.h
+++ b/ipc/ipc_channel_reader.h
@@ -75,6 +75,10 @@ class IPC_EXPORT ChannelReader : public SupportsAttachmentBrokering,
 
   Listener* listener() const { return listener_; }
 
+  // Subclasses should call this method in their destructor to give this class a
+  // chance to clean up state that might be dependent on subclass members.
+  void CleanUp();
+
   // Populates the given buffer with data from the pipe.
   //
   // Returns the state of the read. On READ_SUCCESS, the number of bytes