Add one or more tests to verify that freeing EME objects doesn't cause crashes (see  issue 597355 ).

The original issue was that close() was taking a long time to complete so that it's promise was still pending, and the page was closed freeing all of the EME objects. With the EME objects gone the CDM was freed, which attempted to reject the outstanding close() promise.

This is currently challenging to do in Chromium as the CDM is loaded as a plugin and thus destroyed asynchronously. However, once we switch away from plugins and use the CDM directly, this should be reproducible.