IndexedDB lab code examples update

**Page Affected:** https://developers.google.com/web/ilt/pwa/lab-indexeddb

**What needs to be done?**

Replace `var`s with `const` and `let` in the code examples.
Replace callback function definitions `function () {}` with arrow functions `() => {}`

- [ ] Appropriate `type-something` label has been added.
- [ ] Appropriate `section-something` label has been added.

