IPC: Fix attachment brokering race condition.

ipc/ipc_endpoint.h

 // Copyright 2015 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_ENDPOINT_H_
 #define IPC_IPC_ENDPOINT_H_
 
 #include "base/process/process_handle.h"
 #include "ipc/ipc_export.h"
 #include "ipc/ipc_sender.h"
@@ skipping 14 matching lines @@
   // for race conditions. You can easily get a channel to another process, but
   // if your process has not yet processed the "hello" message from the remote
   // side, this will fail. You should either make sure calling this is either
   // in response to a message from the remote side (which guarantees that it's
   // been connected), or you wait for the "connected" notification on the
   // listener.
   virtual base::ProcessId GetPeerPID() const = 0;
 
   // A callback that indicates that is_attachment_broker_endpoint() has been
   // changed.
-  virtual void OnSetAttachmentBrokerEndpoint(){};
+  virtual void OnSetAttachmentBrokerEndpoint() = 0;
 
   // Whether this channel is used as an endpoint for sending and receiving
   // brokerable attachment messages to/from the broker process.
   void SetAttachmentBrokerEndpoint(bool is_endpoint);
 
  protected:
   bool is_attachment_broker_endpoint() { return attachment_broker_endpoint_; }
 
  private:
   // Whether this channel is used as an endpoint for sending and receiving
   // brokerable attachment messages to/from the broker process.
   bool attachment_broker_endpoint_;
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_ENDPOINT_H_