Source

File: wp-includes/class-wp-customize-manager.php

public function _save_starter_content_changeset() {

		if ( empty( $this->pending_starter_content_settings_ids ) ) {
			return;
		}

		$this->save_changeset_post(
			array(
				'data'            => array_fill_keys( $this->pending_starter_content_settings_ids, array( 'starter_content' => true ) ),
				'starter_content' => true,
			)
		);
		$this->saved_starter_content_changeset = true;

		$this->pending_starter_content_settings_ids = array();
	}