[EME] Bugs for discussion this week: key not available behavior and when to fire needkey events

Below are the issues and bugs that I propose we discuss in the telecon this
week. Since these are complex topics with a lot of previous discussion in
the bugs, please read the bugs for background before the telecon. Feel free
to comment in the bugs or start a thread to discuss them ahead of the
telecon.

*What behavior and event should occur when the key for an encrypted block
is not available?*

   - First, what should the media element do when the key for one of its
   stream(s) is not available?
   https://www.w3.org/Bugs/Public/show_bug.cgi?id=18515
   - Second, should an event be fired in this case?
   https://www.w3.org/Bugs/Public/show_bug.cgi?id=19788 I propose not
   specifying an event in this case, at least until we have use cases:
   https://www.w3.org/Bugs/Public/show_bug.cgi?id=19788#c8


*Should the user agent automatically avoid firing events, creating
sessions, or generating key request messages when the key(s) are already
known or the Initialization Data has already been encountered?*

   - Should a needkey event be fired?
   https://www.w3.org/Bugs/Public/show_bug.cgi?id=16553
   - Should createSession() avoid generating a message?
   https://www.w3.org/Bugs/Public/show_bug.cgi?id=19208

In both cases, I propose that we keep things simple and let the application
or server optimize sessions, requests, etc. as appropriate for each
application.