[EME] Spec updates for the last week

Below is a list of spec updates made in the last week. See
https://dvcs.w3.org/hg/html-media/shortlog for the full list of changes.


   - Implemented Bug 23619 - Change MediaKeyError to extend DOMError and
   use strings for error names
   - Removed the MediaKeyError codes. (A new list of strings will be added
   when Bug 21798 is fixed.)
   - Fixed Bug 24227 - Rename closed event to close
   - release() always schedules a task (related to Bug 17750).
   - Removed "Add the new object to an internal list of session objects"
   steps.
   - Updated DOM4 references to point to the TR/ URL.
   - Restored a minimal set of styles from video-working-draft.css to
   restore important formatting, (mainly affects tables, IDL boxes, and
   examples)
   - Miscellaneous minor fixes, cleanup, parameter checking, etc.