Update ChannelReader to use AttachmentBroker.

ipc/attachment_broker.cc

+// 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.
+
+#include "ipc/attachment_broker.h"
+
+#include <algorithm>
+
+namespace IPC {
+
+AttachmentBroker::AttachmentBroker() {
+}
+AttachmentBroker::~AttachmentBroker() {
+}
+
+void AttachmentBroker::AddObserver(AttachmentBroker::Observer* observer) {
+  auto it = std::find(observers_.begin(), observers_.end(), observer);
+  if (it == observers_.end())
+    observers_.push_back(observer);
+}
+
+void AttachmentBroker::RemoveObserver(AttachmentBroker::Observer* observer) {
+  auto it = std::find(observers_.begin(), observers_.end(), observer);
+  if (it != observers_.end())
+    observers_.erase(it);
+}
+
+void AttachmentBroker::NotifyObservers(
+    const BrokerableAttachment::AttachmentId& id) {
+  // Make a copy of observers_ to avoid mutations during iteration.
+  std::vector<Observer*> observers = observers_;
+  for (Observer* observer : observers) {
+    observer->ReceivedBrokerableAttachmentWithId(id);
+  }
+}
+
+}  // namespace IPC