RFR(S): 8189666: Replace various inlined percentage calculations with global percent_of()

[Thomas Schatzl]

Hi all,

  could I have reviews for this change that replaces some ad-hoc
percentage calculations by the global percent_of() method in gc code?

CR:
https://bugs.openjdk.java.net/browse/JDK-8189666
Webrev:
http://cr.openjdk.java.net/~tschatzl/8189666/webrev/
Testing:
jprt

Thanks,
  Thomas