Add idle play state

Relevant blink patches are

https://codereview.chromium.org/548223002/
https://codereview.chromium.org/555063002/

This is ready for review @shans @dstockwell 
This seems odd, and possibly was already. Perhaps we can remove these two lines and do something like this below:

    ...timeline._players.push...
    ticking = timeline._players.length > 0;
    if (ticking...