The updated version of the asset, containing only the new values of the fields to be updated. Only the
"start_time", "end_time", and "properties" fields can be updated. If a value is named in "updateMask", but is unset in "asset", then that value will be deleted from the asset.
A list of the field names to update. This may contain:
"start_time" or "end_time" to update the corresponding timestamp,
"properties.PROPERTY_NAME" to update a given property, or
"properties" to update all properties. If the list is empty, all properties and both timestamps will be updated.
An optional callback. If not supplied, the call is made synchronously.