8137082: Factor out G1 prediction code from G1CollectorPolicy and clean up Summary: Factor out G1 prediction code from G1CollectorPolicy into its own class, constify methods of G1CollectorPolicy and move more implementations to the cpp file. Reviewed-by: jmasa, sangheki, ecaspole, kbarrett

8138762: Refactor setup of evacuation closures in G1 Summary: Introduce policy class containing the root scan closures. Reviewed-by: ehelin, stefank