Improve support for multiselect list box accessibility on Windows.

base/win/iunknown_impl.h

Index: base/win/iunknown_impl.h
===================================================================
--- base/win/iunknown_impl.h	(revision 0)
+++ base/win/iunknown_impl.h	(revision 0)
@@ -0,0 +1,40 @@
+// Copyright (c) 2011 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.
+
+#ifndef BASE_WIN_IUNKNOWN_IMPL_H_
+#define BASE_WIN_IUNKNOWN_IMPL_H_
+#pragma once
+
+#include <unknwn.h>
+
+#include "base/atomic_ref_count.h"
+#include "base/base_export.h"
+#include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+
+namespace base {
+namespace win {
+
+// IUnknown implementation for other classes to derive from.
+class BASE_EXPORT IUnknownImpl : public IUnknown {
+ public:
+  IUnknownImpl();
+
+  virtual ULONG STDMETHODCALLTYPE AddRef() OVERRIDE;
+  virtual ULONG STDMETHODCALLTYPE Release() OVERRIDE;
+
+  // Subclasses should extend this to return any interfaces they provide.
+  virtual STDMETHODIMP QueryInterface(REFIID riid, void** ppv) OVERRIDE;
+
+ protected:
+  virtual ~IUnknownImpl();
+
+ private:
+  AtomicRefCount ref_count_;
+};
+
+}  // namespace win
+}  // namespace base
+
+#endif  // BASE_WIN_IUNKNOWN_IMPL_H_