[web-animations] Add issue about player sequence number being based on creation time, not attachment time

Original patch by Brian Birtles <birtles@gmail.com>
https://dvcs.w3.org/hg/FXTF/rev/88d33e0ef823
[web-animations] Add description of unresolved time values

Original patch by Brian Birtles <birtles@gmail.com>
https://dvcs.w3.org/hg/FXTF/rev/3e864b57e74f