Generate placeholder content for ListItemBase Field types

Follow-up to #2320157: Generate placeholder content for Field types

Problem/Motivation

Now that #2238085: [regression] options_allowed_values() signature doesn't allow for Views filter configuration has landed, we should be able to implement the ListItemBase::generateSampleValue() method.

Steps to reproduce

1. Install Drupal standard
2. Download and install Devel Generate (part of Devel)
3. Add a field on standard pages
   • Name: whatever
   • Type: any type that starts with "List" eg "List (text)"
   • Allowed values list: at least one value
   • Allowed number of values: whatever
4. Generate standard pages using Devel Generate
5. Edit one of those pages and see that your field has not been filled

Proposed resolution

Adapt the code from Devel Generate 7.x

function list_devel_generate($object, $field, $instance, $bundle) {
  if (field_behaviors_widget('multiple values', $instance) == FIELD_BEHAVIOR_CUSTOM) {
    return devel_generate_multiple('_list_devel_generate', $object, $field, $instance, $bundle);
  }
  else {
    return _list_devel_generate($object, $field, $instance, $bundle);
  }
}

function _list_devel_generate($object, $field, $instance, $bundle) {
  $object_field = array();
  if ($allowed_values = list_allowed_values($field)) {
    $keys = array_keys($allowed_values);
    $object_field['value'] = $keys[mt_rand(0, count($allowed_values) - 1)];
  }
  return $object_field;
}

Remaining tasks

User interface changes

None.

API changes

None.

Why this should be an RC target

That code is never directly used in Core except for tests. It's not disruptive at all. It's going to be used by Devel and enlight #2590993: Create stub entities with proper default values (see #7)