[EME] Spec updates published - update() and isTypeSupported()

Summary of changes:

   - addKey() is now update(). (Bug
18531<https://www.w3.org/Bugs/Public/show_bug.cgi?id=18531>
   )
   - The HTMLMediaElement.canPlayType() extension has been replaced with
   MediaKeys.isTypeSupported(), which matches MSE. (Bug
20335<https://www.w3.org/Bugs/Public/show_bug.cgi?id=20335>
   )
      - There is still an open question of whether two states is enough, so
      this bug remains open.