Update ChannelReader to use AttachmentBroker.

ipc/attachment_broker.cc

Index: ipc/attachment_broker.cc
diff --git a/ipc/attachment_broker.cc b/ipc/attachment_broker.cc
new file mode 100644
index 0000000000000000000000000000000000000000..195b5c72660ca9ef415f4febce3158fff148b2a5
--- /dev/null
+++ b/ipc/attachment_broker.cc
@@ -0,0 +1,37 @@
+// 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"
+
+namespace IPC {
+
+AttachmentBroker::AttachmentBroker() {
+}
+AttachmentBroker::~AttachmentBroker() {
+}
+
+void AttachmentBroker::AddObserver(AttachmentBroker::Observer* observer) {

[Tom Sepez, 2015/07/13 18:28:47]

do we care if |observer| has already been added?

[erikchen, 2015/07/17 18:44:00]

I added logic to prevent the same observer from being added twice.

+  observers_.push_back(observer);
+}
+
+void AttachmentBroker::RemoveObserver(AttachmentBroker::Observer* observer) {
+  for (std::vector<Observer*>::iterator it = observers_.begin();

[Tom Sepez, 2015/07/13 18:28:47]

can we use ::find here?  How about auto keyword to avoid complicated type name

[erikchen, 2015/07/17 18:44:00]

Used auto and find.

+       it != observers_.end(); ++it) {
+    if (*it == observer) {
+      observers_.erase(it);
+      break;
+    }
+  }
+}
+
+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