RFR: 8154051: Change G1YoungGenSizer to use UL log_warning instead of warning

[Stefan Johansson]

Hi,

Please review this small enhancement to change warning to log_warning in 
G1YoungGenSizer:
https://bugs.openjdk.java.net/browse/JDK-8154051

Webrev:
http://cr.openjdk.java.net/~sjohanss/8154051/hotspot.00/

Testing:
Ran the GC jtreg tests and they all pass.

Thanks,
Stefan