Throw exceptions if iterationStart or iterationCount are negative
Throw exceptions if iterationStart or iterationCount are negative

This is required by the spec. It also fixes bugs whereby in some places the value was forced to be non-negative and in other places it was not.

dstockwell/web-animations-js,beni55/web-animations-js-legacy,birtles/web-animations-js,web-animations/web-animations-js-legacy,web-animations/web-animations-js-legacy,beni55/web-animations-js-legacy,web-animations/web-animations-js-legacy,shans/web-anim-js,dstockwell/web-animations-js,birtles/web-animations-js,beni55/web-animations-js-legacy,birtles/web-animations-js,beni55/web-animations-js-legacy