IndexedDB (legacy) LevelDB backing store

Bug: https://crbug.com/40253999

Chromium is in the process of migrating off of the LevelDB implementation and to a SQLite-based replacement. This virtual test suite replaces the former virtual suite that force-enabled the SQLite store, which was called indexeddb_sqlite.

The default in Chromium is currently (still) to use LevelDB, but due to an experimental entry in testing/variations/fieldtrial_testing_config.json, the default for WPT is SQLite. Therefore this virtual suite is now necessary to ensure continued coverage of the LevelDB store during what is expected to be a protracted rollout of the SQLite store.

This virtual test suite is not set to automatically expire, but can be removed when the LevelDB store code is eventually deleted.

The functionality is platform-agnostic but the implementation ultimately uses platform-specific libraries. This suite is currently run only on Windows and Linux to reduce load on bots while still providing coverage across OS families.