Protect AppListItemList Add/Remove and fix sync bugs

ui/app_list/app_list_folder_item.cc

Index: ui/app_list/app_list_folder_item.cc
diff --git a/ui/app_list/app_list_folder_item.cc b/ui/app_list/app_list_folder_item.cc
index 1aa14af5ea65a4d76a5ec7002c7f8cb0d1c30d66..a20fbcdab58fe15e07b8d6678af1680d7f519757 100644
--- a/ui/app_list/app_list_folder_item.cc
+++ b/ui/app_list/app_list_folder_item.cc
@@ -4,6 +4,7 @@
 
 #include "ui/app_list/app_list_folder_item.h"
 
+#include "base/guid.h"
 #include "ui/app_list/app_list_constants.h"
 #include "ui/app_list/app_list_item_list.h"
 #include "ui/gfx/canvas.h"
@@ -144,6 +145,10 @@ Rects AppListFolderItem::GetTopIconsBounds(
   return top_icon_bounds;
 }
 
+std::string AppListFolderItem::GenerateId() {
+  return base::GenerateGUID();
+}
+
 const char* AppListFolderItem::GetItemType() const {
   return AppListFolderItem::kItemType;
 }