Samples should indicate which are usable across browsers

We're likely a long way from needing to do this, but I want to capture I while it's in my head.

As the browser vendors converge on an extensions spec, extension developer will inevitably ask which of our extensions works across browsers. When that stars, we should:

- [ ] Label all samples as cross-browser or Chrome only
- [ ] Surface this information in the [samples search page](https://developer.chrome.com/docs/extensions/samples/)