Add a SharedMemSynchronizer class.

media/audio/shared_mem_synchronizer.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/shared_mem_synchronizer.h"
+
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+
+SharedMemSynchronizer::SharedMemSynchronizer() {}
+
+SharedMemSynchronizer::WaitForMultiple::WaitForMultiple(
+    const SynchronizerVector* synchronizers)
+    : synchronizers_(synchronizers), last_(synchronizers_->size() - 1) {

[Ami GONE FROM CHROMIUM, 2012/03/13 20:08:02]

Why init last_ (an unsigned type) to the result of a subtraction (always a
sketchy thing to do with unsigned types)?
IOW doing last_(0) would be no less correct, but less confusing.

However, using last_(0) has the down-side of the unobvious fact that the first
run through WaitMultiple will populate [1,size) followed by 0.
If instead of tracking last_ you tracked next_start_offset_ then you could drop
the -1 here (by  initializing to 0 here), and have a more expected order in the
first run of WaitMultiple (and subsequent runs stay as obvious).  Plus, IMO, the
calc of 
(i + next_wait_offset_) % synchronizers.size()
is slightly clearer than 
(i + last_signaled + 1) % synchronizers.size()

[tommi (sloooow) - chröme, 2012/03/14 13:32:43]

Done.

+  DCHECK_GT(synchronizers_->size(), 0U);

[Ami GONE FROM CHROMIUM, 2012/03/13 20:08:02]

clearer as 
DCHECK(!synchronizers_->empty());

[tommi (sloooow) - chröme, 2012/03/14 13:32:43]

Done.

+}
+
+int SharedMemSynchronizer::WaitForMultiple::Wait() {
+  int ret = WaitMultiple(*synchronizers_, last_);
+  last_ = static_cast<size_t>(ret);
+  return ret;
+}
+
+void SharedMemSynchronizer::WaitForMultiple::Reset(
+    const SynchronizerVector* synchronizers) {
+  synchronizers_ = synchronizers;
+  DCHECK_GT(synchronizers_->size(), 0U);

[Ami GONE FROM CHROMIUM, 2012/03/13 20:08:02]

ditto

[Ami GONE FROM CHROMIUM, 2012/03/13 20:08:02]

reset next_start_offset_ (nee last_)?

[tommi (sloooow) - chröme, 2012/03/14 13:32:43]

Done.

[tommi (sloooow) - chröme, 2012/03/14 13:32:43]

Done.

+}