RFR: 8149820: Move G1YoungGenSizer to g1CollectorPolicy.cpp

[Erik Helin]

Hi all,

this small patch just moves the class G1YoungGenSizer from
g1CollectorPolicy.hpp to g1CollectorPolicy.cpp. No code outside of
g1CollectorPolicy.cpp is using G1YoungGenSizer and we want to keep it
that way :)

Webrev:
http://cr.openjdk.java.net/~ehelin/8149820/00/

Testing:
- JPRT

Thanks,
Erik