[EME] Spec updates: common formats; Clear Key

Spec updates this week include:

   - Common SystemID and PSSH Box Format
   <https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/cenc-format.html#common-system>
for
   CENC (bug 24027 <https://www.w3.org/Bugs/Public/show_bug.cgi?id=24027>)
   - Key IDs Initialization Data Format
   <https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/keyids-format.html>
    (bug 25269 <https://www.w3.org/Bugs/Public/show_bug.cgi?id=25269>)
   - Rewrite of the Clear Key
   <https://dvcs.w3.org/hg/html-media/raw-file/default/encrypted-media/encrypted-media.html#clear-key>
   definition, including license request and license formats (bugs 17682
   <https://www.w3.org/Bugs/Public/show_bug.cgi?id=17682>, 24419
   <https://www.w3.org/Bugs/Public/show_bug.cgi?id=24419>)