In order to require explicit enabling of EME in nested contexts (https://github.com/w3c/encrypted-media/issues/364), we would need to deprecate the current implicit enabling. In order to do that, we'd need metrics to tell us that pages aren't relying on it.

We should at least track when EME is used in an embedded context/iframe. It might be useful to separately track when uses where the origin is different from the top level context. If we think there will be a lot of uses that would actually show up, we could add RAPPOR, but I don't expect that to be the case, and that would require handling this in Chromium rather than Blink.