Rough draft of EME Concerns document.

As discussed in various forums, this is a version of the opinion document that tries to catalog our concerns for EME as a web-facing API, without suggesting solutions or focusing on specific technical choices or problems with the current mechanism. Feedback definitely welcome.

If we say that codec is an analogue of KeySystem, then these two paragraphs contradicts each other, since previous one says that content provider shouldn't be able to restrict content to certain KeySystem.