Update ChannelReader to use AttachmentBroker.

ipc/attachment_broker.h

Index: ipc/attachment_broker.h
diff --git a/ipc/attachment_broker.h b/ipc/attachment_broker.h
index 6bee1b29643aef464132ad9d6decb15ee8c5afb6..efde3c132ead007b978ec14bd0b64cd9ff8f9ce5 100644
--- a/ipc/attachment_broker.h
+++ b/ipc/attachment_broker.h
@@ -27,10 +27,20 @@ class IPC_EXPORT SupportsAttachmentBrokering {
 // 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
@@ -40,13 +50,22 @@ class IPC_EXPORT AttachmentBroker : public Listener {
                                        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