Improve support for multiselect list box accessibility on Windows.

base/win/iunknown_impl.cc

Index: base/win/iunknown_impl.cc
===================================================================
--- base/win/iunknown_impl.cc	(revision 0)
+++ base/win/iunknown_impl.cc	(revision 0)
@@ -0,0 +1,42 @@
+// 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.
+
+#include "base/win/iunknown_impl.h"
+
+namespace base {
+namespace win {
+
+IUnknownImpl::IUnknownImpl()
+    : ref_count_(0) {
+}
+
+IUnknownImpl::~IUnknownImpl() {
+}
+
+ULONG STDMETHODCALLTYPE IUnknownImpl::AddRef() {
+  base::AtomicRefCountInc(&ref_count_);
+  return 1;
+}
+
+ULONG STDMETHODCALLTYPE IUnknownImpl::Release() {
+  if (!base::AtomicRefCountDec(&ref_count_)) {
+    delete this;
+    return 0;
+  }
+  return 1;
+}
+
+STDMETHODIMP IUnknownImpl::QueryInterface(REFIID riid, void** ppv) {
+  if (riid == IID_IUnknown) {
+    *ppv = static_cast<IUnknown*>(this);
+    AddRef();
+    return S_OK;
+  }
+
+  *ppv = NULL;
+  return E_NOINTERFACE;
+}
+
+}  // namespace win
+}  // namespace base