ui: Change ListModel to operate in size_t instead of int.

ui/app_list/app_list_model.cc

Index: ui/app_list/app_list_model.cc
diff --git a/ui/app_list/app_list_model.cc b/ui/app_list/app_list_model.cc
index 529ed161a594385edacc57d66703ee2f8ddb9996..f0f8f115f81ea45f9f1616fee08f7fe1b79b60af 100644
--- a/ui/app_list/app_list_model.cc
+++ b/ui/app_list/app_list_model.cc
@@ -16,15 +16,15 @@ void AppListModel::AddItem(AppListItemModel* item) {
   items_.Add(item);
 }
 
-void AppListModel::AddItemAt(int index, AppListItemModel* item) {
+void AppListModel::AddItemAt(size_t index, AppListItemModel* item) {
   items_.AddAt(index, item);
 }
 
-void AppListModel::DeleteItemAt(int index) {
+void AppListModel::DeleteItemAt(size_t index) {
   items_.DeleteAt(index);
 }
 
-AppListItemModel* AppListModel::GetItemAt(int index) {
+AppListItemModel* AppListModel::GetItemAt(size_t index) {
   return items_.GetItemAt(index);
 }