Add warning comments regarding downgrades am: 3e720972 am: 8fa830ac am: c244da48

    /** The SQLite DB helper */

    public static class DbHelper extends SQLiteOpenHelper {

        // Update this whenever changing the schema.

        // DO NOT CHANGE without solid testing for downgrades, and checking onDowngrade

        // below: b/171340630

        private static final int SCHEMA_VERSION = 4;

        private static final String DATABASE_FILENAME = "IpMemoryStore.db";

        private static final String TRIGGER_NAME = "delete_cascade_to_private";

            // Downgrades always nuke all data and recreate an empty table.

            db.execSQL(NetworkAttributesContract.DROP_TABLE);

            db.execSQL(PrivateDataContract.DROP_TABLE);

            // TODO: add test for downgrades. Triggers should already be dropped

            // when the table is dropped, so this may be a bug.

            // Note that fixing this code does not affect how older versions

            // will handle downgrades.

            db.execSQL("DROP TRIGGER " + TRIGGER_NAME);

            onCreate(db);

        }