[web-animations-2] Allow negative infinity start time / delay

[birtles]

From @birtles on February 3, 2015 6:50

See thread starting here:
https://lists.w3.org/Archives/Public/public-fx/2014OctDec/0011.html

Copied from original issue: w3c/web-animations#48

[birtles]

I suspect we may not end up doing this but if we do it might affect the API.

[birtles]

Moving this to level 2 since I haven't seen any solid use cases for this.