Re: [selection-api] Make some arguments optional (#30)

I've submitted a pull request to make it unambiguous, PTAL.

The fate of `setBaseAndExtent` will be determined by this use counter which has not yet reached the stable channel: https://www.chromestatus.com/metrics/feature/timeline/popularity/1084

And then there's also the `modify` method that is the topic of #37

I think it's fine to close this issue with the current PR merged though, I'll open a new issue if anything else needs to be optional later.

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