Factor out TimedItem._getAdjustedAnimationTime()

This is to simplify implementing a fix for
https://github.com/web-animations/web-animations-js/issues/39
Factor out TimedItem._scaleIterationTime()

This is to simplify implementing a fix for
https://github.com/web-animations/web-animations-js/issues/39
Factor out TimedItem._getEffectiveSpeed()

This is to simplify implementing a fix for
https://github.com/web-animations/web-animations-js/issues/39
Remove broken TimedItem._parentToGlobalTime() and replace with new Animation._inheritedTimeToDocumentTime()

This fixes https://github.com/web-animations/web-animations-js/issues/39
Add tests for compositing order

See https://github.com/web-animations/web-animations-js/issues/39