RFR: 8146990: Convert CollectorPolicy to use log_warning instead of warning

[Erik Helin]

Hi all,

this patchs updates CollectorPolicy to use log_warning(gc, ergo) instead of
warning (as part of updating the GC code to use unified logging).

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8146990

Webrev:
http://cr.openjdk.java.net/~ehelin/8146990/00/webrev/

Testing:
- JPRT

Thanks,
Erik