Factor out TimedItem._getEffectiveSpeed()
Factor out TimedItem._getEffectiveSpeed()

This is to simplify implementing a fix for
https://github.com/web-animations/web-animations-js/issues/39

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