Migrate BrowserAccessibility windows unique id handling to AXPlatformNodeWin.

ui/accessibility/platform/ax_platform_node.cc

 // Copyright 2014 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 "ui/accessibility/platform/ax_platform_node.h"
 
 #include "base/containers/hash_tables.h"
 #include "base/lazy_instance.h"
 #include "build/build_config.h"
 #include "ui/accessibility/ax_node_data.h"
@@ skipping 17 matching lines @@
 AXPlatformNode::~AXPlatformNode() {
   if (unique_id_)
     g_unique_id_map.Get().erase(unique_id_);
 }
 
 void AXPlatformNode::Destroy() {
   g_unique_id_map.Get().erase(unique_id_);
   unique_id_ = 0;
 }
 
+// static
 AXPlatformNode* AXPlatformNode::GetFromUniqueId(int32_t unique_id) {
   UniqueIdMap* unique_ids = g_unique_id_map.Pointer();
   auto iter = unique_ids->find(unique_id);
   if (iter != unique_ids->end())
     return iter->second;
 
   return nullptr;
 }
 
+int32_t AXPlatformNode::unique_id() const {
+  return unique_id_;
+}
+
 }  // namespace ui