Reland #1: Clean up interface of attachment broker.

ipc/attachment_broker_unprivileged.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_ATTACHMENT_BROKER_UNPRIVILEGED_H_
 #define IPC_ATTACHMENT_BROKER_UNPRIVILEGED_H_
 
 #include "ipc/attachment_broker.h"
 #include "ipc/ipc_export.h"
 
 namespace IPC {
 
+class Channel;
 class Sender;
 
 // This abstract subclass of AttachmentBroker is intended for use in
 // non-privileged processes.
 class IPC_EXPORT AttachmentBrokerUnprivileged : public IPC::AttachmentBroker {
  public:
   AttachmentBrokerUnprivileged();
   ~AttachmentBrokerUnprivileged() override;
 
-  void set_sender(IPC::Sender* sender) { sender_ = sender; }
+  // In each unprivileged process, exactly one channel should be used to
+  // communicate brokerable attachments with the broker process.
+  void DesignateBrokerCommunicationChannel(IPC::Channel* channel);
 
  protected:
   IPC::Sender* get_sender() { return sender_; }
 
  private:
   // |sender_| is used to send Messages to the privileged broker process.
   // |sender_| must live at least as long as this instance.
   IPC::Sender* sender_;
   DISALLOW_COPY_AND_ASSIGN(AttachmentBrokerUnprivileged);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_ATTACHMENT_BROKER_UNPRIVILEGED_H_