Add sample code for single controller to multiple presentations

[markafoltz]

Per https://www.w3.org/2018/05/17-webscreens-minutes.html#x03, we don't have a code snippet demonstrating that one controlling page can start presentations on two (or more) distinct presentation displays.

This is called out as a specific use case and it would be good to add sample code for it.

[markafoltz]

I also filed web-platform-tests/wpt#11233 as a task to add the corresponding WPT.

[markafoltz]

Closed with PR #453.