If we choose shared LevelDB USS implementation:

- Implement ModelType::WEB_APPS
(follow https://chromium.googlesource.com/chromium/src/+/HEAD/docs/sync/model_api.md#sync-integration-checklist
in general)

- Move all .proto files into components/sync/protocol/
- Use shared ModelTypeStoreService from profile

See comments here: https://chromium-review.googlesource.com/c/chromium/src/+/1295529