refactor internal representation of length to take into account percent as well; support mixing percent and pixel via -webkit-calc.
refactor internal representation of length to take into account percent as well; support mixing percent and pixel via -webkit-calc.

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