we should have some tests for copy & paste behavior to make sure it works, and we don't allow things we shouldn't.  specifically:
- copying in Chrome browser and pasting into focused Crostini app works
- copying in Crostini app and pasting into focused Chrome browser works
- copying in Chrome browser and a non-focused/background Crostini app *cannot* paste data
- a non-focused/background Crostini app *cannot* copy data and have a Chrome browser paste it

for background, check out this post:
  https://developers.google.com/web/updates/2018/03/clipboardapi#security_and_permissions

tentatively sending to Jeff as he's doing test stuff currently.