RFR (S/M): 8142484: Let IHOP follow the current heap size, not the maximum heap size

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that updates what maximum
threshold (capacity) the IHOP mechanisms follow?

Instead of following the maximum capacity, IHOP follows the current
capacity now.

Changes are relatively small, instead of passing a fixed threshold
capacity at initialization, add and use a method in the G1IHOPControl
class to update to the current threshold.

CR:
https://bugs.openjdk.java.net/browse/JDK-8142484
Webrev:
cr.openjdk.java.net/~tschatzl/8142484/webrev
Testing:
jprt, local testing that it works, vm.gc run

Thanks,
  Thomas