Could we load `pb` in a custom loader to avoid deprecated message?
With something like this:
```
let { Loader } = require('sdk/test/loader');
let hookedConsole = Object.create(console, {
    error: {
      value: function(e) {
        if (!/DEPRECATED:/.test(e)) {
          console.error(e);
        }
      }
    }
  });
let loader = LoaderWithHookedConsole(module, {console: hookedConsole});
const pb = loader.require('sdk/private-browsing');
```