RFR: 8151365: Move G1YoungGenSizer to a separate file

[Erik Helin]

Hi all,

this small patch just moves the class G1YoungGenSizer to a separate
file. I will send out an additional patch that changes most of the
methods to be const, but in this patch I just want to move the code.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8151365

Webrev:
http://cr.openjdk.java.net/~ehelin/8151365/webrev.00/

Testing:
- Built locally on Linux x86-64

Thanks,
Erik