Revert of Clean up public interface of AttachmentBrokerUnprivileged.

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 "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "ipc/attachment_broker.h"
 #include "ipc/ipc_export.h"
 
 namespace IPC {
 
 class Endpoint;
 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;
 
-  // If there is no global attachment broker, makes a new
-  // AttachmentBrokerUnprivileged and sets it as the global attachment broker.
-  // This method is thread safe.
-  static void CreateBrokerIfNeeded();
+   // On platforms that support attachment brokering, returns a new instance of
+   // a platform-specific attachment broker. Otherwise returns |nullptr|.
+   // The caller takes ownership of the newly created instance, and is
+   // responsible for ensuring that the attachment broker lives longer than
+   // every IPC::Channel. The new instance automatically registers itself as the
+   // global attachment broker.
+  static scoped_ptr<AttachmentBrokerUnprivileged> CreateBroker();
 
-  // AttachmentBroker:
-  void RegisterBrokerCommunicationChannel(Endpoint* endpoint) override;
-  void DeregisterBrokerCommunicationChannel(Endpoint* endpoint) override;
-  bool IsPrivilegedBroker() override;
+  // In each unprivileged process, exactly one channel should be used to
+  // communicate brokerable attachments with the broker process.
+  void DesignateBrokerCommunicationChannel(Endpoint* endpoint);
 
  protected:
   IPC::Sender* get_sender() { return sender_; }
 
   // Errors that can be reported by subclasses.
   // These match tools/metrics/histograms/histograms.xml.
   // This enum is append-only.
   enum UMAError {
     // The brokerable attachment was successfully processed.
     SUCCESS = 0,
@@ skipping 11 matching lines @@
  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_