See 
https://docs.google.com/presentation/d/1Z6INkdj61Td0pnqbdelD_wrnHmTZrmdh9SEuhMmgXKI/edit#slide=id.g4314d3ad89_12_207 for details. 

For a tab that's being shared we want to add a notification border around the shared content. There's should also be a description of which domain that initiated the sharing, and the option to stop the share.

Pending further UX discussions this overlay should be complementary to the existing screen overlay (bottom bar).