**Description**

For a DAG that looks like this:

```
A >> B
```

If A fails, B goes into "upstream_failed" state. If a user then marks A "success", B will remain in "upstream_failed" state. It will not automatically start running. This scenario often happens if the failure of A is dealt with outside of Airflow and the user does not want Airflow to run A again. But he usually expect Airflow to run B after A is marked "success".

**Use case / motivation**

After A is marked "success", its downstream tasks B should be cleared automatically and get ready to be scheduled. To avoid changing this behaviour completely, there can be a toggle button next to "Success" that lets the user decide if he wants Airflow to automatically clear downstream tasks that are in "upstream_failed" state.
airflow/www/views.py
}
class XComModelView(AirflowModelView):
    """View to show records from XCom table"""
    route_base = '/xcom'
    list_title = 'List XComs'
    datamodel = AirflowModelView.CustomSQLAInterface(XCom)
    class_permission_name = permissions.RESOURCE_XCOM
    method_permission_name = {
        'list': 'read',
        'delete': 'delete',
        'action_muldelete': 'delete',
    }
    base_permissions = [
        permissions.ACTION_CAN_CREATE,
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_DELETE,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    search_columns = ['key', 'value', 'timestamp', 'execution_date', 'task_id', 'dag_id']
    list_columns = ['key', 'value', 'timestamp', 'execution_date', 'task_id', 'dag_id']
    base_order = ('execution_date', 'desc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    formatters_columns = {
        'task_id': wwwutils.task_instance_link,
        'execution_date': wwwutils.datetime_f('execution_date'),
        'timestamp': wwwutils.datetime_f('timestamp'),
        'dag_id': wwwutils.dag_link,
    }
    @action('muldelete', 'Delete', "Are you sure you want to delete selected records?", single=False)
    def action_muldelete(self, items):