Migrate BrowserAccessibility windows unique id handling to AXPlatformNodeWin.

ui/accessibility/platform/ax_platform_node.cc

Index: ui/accessibility/platform/ax_platform_node.cc
diff --git a/ui/accessibility/platform/ax_platform_node.cc b/ui/accessibility/platform/ax_platform_node.cc
index 93a374abbacd9dd610e47b1aa8d1dd64803fbe17..8635ff03c5e5898751bfc43c57ce0466e49b00d6 100644
--- a/ui/accessibility/platform/ax_platform_node.cc
+++ b/ui/accessibility/platform/ax_platform_node.cc
@@ -35,6 +35,7 @@ void AXPlatformNode::Destroy() {
   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);
@@ -44,4 +45,8 @@ AXPlatformNode* AXPlatformNode::GetFromUniqueId(int32_t unique_id) {
   return nullptr;
 }
 
+int32_t AXPlatformNode::unique_id() const {
+  return unique_id_;
+}
+
 }  // namespace ui