[EME] *mp4-unique-origin.html doesn't enforce anything in the EME spec

clearkey-mp4-unique-origin.html, and its DRM variant, test that EME can't be used on a "unique origin". I don't see anything in the current EME spec that says that a unique origin can't use EME.

Technically, there's no reason why EME can't work on a unique origin. It would seem this test is testing Chrome specific behaviour? This test came from the Google test suite.

However, the spec does say that EME is only usable inside a "secure context", and as per the Secure Contexts spec a data: URI inside a sandboxed iframe is not considered secure. But there is disagreement as to whether a data: URI inside a sandboxed iframe should be considered an insecure context, see Secure Context Spec issue 26.

It does not seem sensible for the EME test suite to be enforcing behaviour that is contested in another unfinished spec.

We should either remove this test, or change it such that it's insulated from the general question as to which contexts are considered secure.

Start with clearkey-mp4-unique-origin.html and its DRM variant, then compare their assumptions with the EME secure-context requirements and Secure Contexts issue 26. Confirm whether the tests enforce behavior absent from EME or dependent on the contested context definition; done means removing the test or insulating it from that unresolved question.