Add a SharedMemSynchronizer class.

media/audio/cross_process_notification.cc

+// Copyright (c) 2012 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 "media/audio/cross_process_notification.h"
+
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+
+CrossProcessNotification::CrossProcessNotification() {}
+
+CrossProcessNotification::WaitForMultiple::WaitForMultiple(
+    const Notifications* notifications)

[Ami GONE FROM CHROMIUM, 2012/03/14 16:40:51]

FWIW the initializer-list & ctor body can both be replaced with a call to
Reset(notifications) now.

[tommi (sloooow) - chröme, 2012/03/14 21:20:38]

Done.

+    : notifications_(notifications), wait_offset_(0) {
+  DCHECK(!notifications_->empty());
+}
+
+int CrossProcessNotification::WaitForMultiple::Wait() {
+  DCHECK(CalledOnValidThread());
+  int ret = WaitMultiple(*notifications_, wait_offset_);
+  wait_offset_ = (static_cast<size_t>(ret) + 1) % notifications_->size();

[Ami GONE FROM CHROMIUM, 2012/03/14 16:40:51]

Is the static_cast<size_t>() really necessary?
(I can't make a toy program that generates a warning/error with either clang or
gcc on my z600, building in either -m32 or -m64).

[tommi (sloooow) - chröme, 2012/03/14 21:20:38]

it's not necessary. removed.

+  return ret;
+}
+
+void CrossProcessNotification::WaitForMultiple::Reset(
+    const Notifications* notifications) {
+  DCHECK(CalledOnValidThread());
+  wait_offset_ = 0;
+  notifications_ = notifications;
+  DCHECK(!notifications_->empty());
+}