frame-clock: Add 'compute-size' phase

frame-clock: Add 'compute-size' phase

This will be handled between 'update' (which may trigger animation
ticks, CSS update, etc) and 'layout' which will allocate the widget
tree. It's meant to perform surface size computation, and is done
between these two phases in order to have an up to date state, and
letting the layout phase have an up to date size to layout in.