When separating the MSGs per docgroup or document, `test_eme_stream_capture_blocked_case3.html` calls `MediaStreamGraph::GetInstance`, and then fails to create a stream because it's not allowed (because of, here, EME), and the MSG sits there doing nothing until the end of the process, and can't shut down because it's not progressed in its life cycle state machine. This is a footgun, and we should try to make it impossible to create an MSG without a stream, but in the meantime, this does the trick.

Comment on attachment 8876192 [details] Bug 1371719 - Don't create a MediaStreamGraph without doing anything with it. https://reviewboard.mozilla.org/r/147634/#review152034

Pushed by paul@paul.cx: https://hg.mozilla.org/integration/mozilla-inbound/rev/1c4d113ff7fb Don't create a MediaStreamGraph without doing anything with it. r=jesup