Update ChannelReader to use AttachmentBroker.

ipc/attachment_broker.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_H_
 #define IPC_ATTACHMENT_BROKER_H_
 
 #include "base/macros.h"
 #include "base/process/process_handle.h"
 #include "ipc/brokerable_attachment.h"
 #include "ipc/ipc_export.h"
 #include "ipc/ipc_listener.h"
 
 namespace IPC {
 
 class AttachmentBroker;
 // Classes that inherit from this abstract base class are capable of
 // communicating with a broker to send and receive attachments to Chrome IPC
 // messages.
 class IPC_EXPORT SupportsAttachmentBrokering {
  public:
   // Returns an AttachmentBroker used to broker attachments of IPC messages to
   // other processes. There must be exactly one AttachmentBroker per process.
   virtual AttachmentBroker* GetAttachmentBroker() = 0;
 };
 
 // Responsible for brokering attachments to Chrome IPC messages. On platforms
 // that support attachment brokering, every IPC channel should have a reference
 // to a AttachmentBroker.
+// This class is not thread safe. The implementation of this class assumes that
+// it is only ever used on the same thread as its consumers.
 class IPC_EXPORT AttachmentBroker : public Listener {
  public:
-  AttachmentBroker() {}
-  ~AttachmentBroker() override {}
+  // A standard observer interface that allows consumers of the AttachmentBroker
+  // to be notified when a new attachment has been received.
+  class Observer {
+   public:
+    virtual void ReceivedBrokerableAttachmentWithId(
+        const BrokerableAttachment::AttachmentId& id) = 0;
+  };
+
+  AttachmentBroker();
+  ~AttachmentBroker() override;
 
   // Sends |attachment| to |destination_process|. The implementation uses an
   // IPC::Channel to communicate with the broker process. This may be the same
   // IPC::Channel that is requesting the brokering of an attachment.
   // Returns true on success and false otherwise.
   virtual bool SendAttachmentToProcess(const BrokerableAttachment* attachment,
                                        base::ProcessId destination_process) = 0;
 
   // Returns whether the attachment was available. If the attachment was
-  // available, populates the output parameter |attachment|. The caller then
-  // becomes the owner of |attachment|.
-  virtual bool GetAttachmentWithId(BrokerableAttachment::AttachmentId id,
-                                   BrokerableAttachment* attachment) = 0;
+  // available, populates the output parameter |attachment|.
+  virtual bool GetAttachmentWithId(
+      BrokerableAttachment::AttachmentId id,
+      scoped_refptr<BrokerableAttachment>* attachment) = 0;
+
+  // Any given observer should only ever add itself once to the observer list.
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+ protected:
+  void NotifyObservers(const BrokerableAttachment::AttachmentId& id);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(AttachmentBroker);
+
+  std::vector<Observer*> observers_;
 };
 
 }  // namespace IPC
 
 #endif  // IPC_ATTACHMENT_BROKER_H_