[EME] Uint8Array parameters may be modified while scheduled task is pending

MediaKeys.createSession() and MediaKeySession.update() both accept
Uint8Arrays. Since these methods schedule a task, it's possible that
JavaScript can modify the contents of those arrays before the task is run.

For example:

> array[0]=0;
> mediaKeys.createSession("video/webm", array);
> array[0]=9;
> // Scheduled task will run later and see 9 in the first element.


I believe there is a similar issue with MSE, and it was decided that such
behavior is undefined. Does anyone object to the behavior also being
undefined for EME? Do we need to explicitly document this anywhere?

David