Termination should also "discard" the receiving browsing context

[tidoust]

The steps to terminate a presentation in a receiving browsing context tell the receiving user agent to unload a document.

I believe the intent is to close the context entirely, but there are a couple of additional steps needed to close a browsing context:

• Remove the specified browsing context from the user interface (e.g., close or hide its tab in a tabbed browser).
• Discard the specified browsing context.

The first point is UA-specific and could perhaps go without saying, but the second point is neeed to set window.closed. A quick test suggests that Chrome already does that correctly.

In other words, I propose to revisit the bullet point we have to "If there is a receiving browsing context for P, and it has a document for P that is not unloaded, unload a document corresponding to that browsing context, remove that browsing context from the user interface and discard it".

[markafoltz]

SGTM