Make attachment broker a subclass of IPC::Listener.

ipc/handle_attachment_win.h

Index: ipc/handle_attachment_win.h
diff --git a/ipc/handle_attachment_win.h b/ipc/handle_attachment_win.h
index 7dd898ad54be0f3440732f2038c425b11fd3424c..5b7528216feb310ce336cba49b5f44767c83e5b8 100644
--- a/ipc/handle_attachment_win.h
+++ b/ipc/handle_attachment_win.h
@@ -31,13 +31,16 @@ class IPC_EXPORT HandleAttachmentWin : public BrokerableAttachment {
     AttachmentId attachment_id;
   };
 
-  HandleAttachmentWin(const HANDLE& handle);
+  explicit HandleAttachmentWin(const HANDLE& handle);
+  explicit HandleAttachmentWin(const WireFormat& wire_format);
 
   BrokerableType GetBrokerableType() const override;
 
   // Returns the wire format of this attachment.
   WireFormat GetWireFormat(const base::ProcessId& destination) const;
 
+  HANDLE get_handle() const { return handle_; }
+
  private:
   ~HandleAttachmentWin() override;
   HANDLE handle_;