RFR (XXS): 8136680: Enable adaptive IHOP by default

[Thomas Schatzl]

Hi all,

  can I have reviews for the following small change that enables
adaptive IHOP by default by setting G1UseAdaptiveIHOP to true by
default.

This feature improves usability of G1 as the user does not need to care
about the InitiatingHeapOccupancyPercent option any more. It will be set
automatically based on predictions of previous application behavior.

The existing InitiatingHeapOccupancyPercent option still serves as
initial value for the predictions.
Of course, by manually setting G1UseAdaptiveIHOP to false, original
behavior is preserved.

CR:
https://bugs.openjdk.java.net/browse/JDK-8136680

Webrev:
http://cr.openjdk.java.net/~tschatzl/8136680/webrev/

Testing:
Previous dependent CRs adding adaptive IHOP functionality. Local build
for this particular change.

Thanks,
  Thomas