[EME] bugs for discussion at the 5/7 telecon

The bugs below will likely form the agenda for the telecon Tuesday. Please
read the bugs and feel free to comment in them before the telecon.

The following are the new bugs mentioned during the last telecon. They
replace multiple older bugs that were focused on subsets of these broader
issues.

   - Define MediaKeySession life cycle states and/or
events<https://www.w3.org/Bugs/Public/show_bug.cgi?id=21854>
   - This one has 4 dependent
bugs<https://www.w3.org/Bugs/Public/showdependencytree.cgi?id=21854>
   - Avoid network traffic and duplicate sessions for the same
key(s)<https://www.w3.org/Bugs/Public/show_bug.cgi?id=21855>
   - Revisit MediaKeyError
codes<https://www.w3.org/Bugs/Public/show_bug.cgi?id=21798>


In addition, I'd like to close out the recent discussion in these bugs.

   - Revisit causes of
MEDIA_ERR_ENCRYPTED<https://www.w3.org/Bugs/Public/show_bug.cgi?id=16857>
   - Provide more details on behavior of the media element when the key for
   an encrypted block is not
available<https://www.w3.org/Bugs/Public/show_bug.cgi?id=18515>
   - Add 'type' attribute to
MediaKeyNeededEvent<https://www.w3.org/Bugs/Public/show_bug.cgi?id=19096>
   - Revert addition of keySystem attribute to
HTMLSourceElement<https://www.w3.org/Bugs/Public/show_bug.cgi?id=20336>
   - Should createSession()'s type parameter be
required?<https://www.w3.org/Bugs/Public/show_bug.cgi?id=20691>