RFR(XXS) 8209852: Counters in StringCleaningTask should be type of size_t

[Zhengyu Gu]

Hi,

Please review this trivial change that converts counters from type int 
to size_t, it eliminates several unnecessary type casting.

Bug: https://bugs.openjdk.java.net/browse/JDK-8209852
Webrev: http://cr.openjdk.java.net/~zgu/8209852/webrev.00/index.html

Test:

   hotspot_gc on Linux 64 (fastdebug and release)


Thanks,

-Zhengyu