RFR (S): 8248322: G1: Refactor full collection sizing code

[Thomas Schatzl]

Hi all,

   can I have reviews for this small refactoring changes that moves some 
heap sizing policy related code into a the "right" place (into the heap 
sizing policy class), and refactors it a bit.

CR:
https://bugs.openjdk.java.net/browse/JDK-8248322
Webrev:
http://cr.openjdk.java.net/~tschatzl/8248322/webrev/
Testing:
tier1

Thanks,
   Thomas