RFR (XS): 8073654: Marking statistics should use size_t

[Sangheon Kim]

Hi all,

Can I have reviews for this simple fix for changing variables for 
statistics from 'int' to 'size_t'.
This is to avoid overflows on large heaps.

CR:
https://bugs.openjdk.java.net/browse/JDK-8073654
Webrev:
http://cr.openjdk.java.net/~sangheki/8073654/webrev.00/
Testing:
JPRT

Thanks,
Sangheon