This introduces an AnimationPerformance class, which is intended to manage an
animation (or its reverse), with the ability to manually control the timeline
or to apply a force to advance the animation with a diminishing speed.

I'm having trouble fitting the odeon model to Sky. Odeon has a lot of nice
properties, but fundamentally operates on UINodes, which contain all the
properties to be animated. Sky, on the other hand, has no such universal
properties. Instead, each Widget assembles itself how it sees fit.

So my current plan is to let AnimationPerformance own a generic set of
AnimatedVariables. You pass it a bag of things, say position and opacity, as
AnimatedVariables. It updates them based on the animation, and they each have
a way to build a widget based on their current state.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1211603003.