CRR (XXS): 7146246: G1: expose some of the -XX flags that drive which old regions to collect during mixed GCs

[Tony Printezis]

Hi all,

Can I have a couple of quick code review for this very small change?

http://cr.openjdk.java.net/~tonyp/7146246/webrev.0/

It turns the following two cmd line params into product params:

G1HeapWastePercent (previously called: G1OldReclaimableThresholdPercent)
G1MixedGCCountTarget (previous called: G1MaxMixedGCNum)

These are helpful in allowing the user to fine-tune G1's behavior wrt 
which old regions to collect during mixed GCs, how many mixed GCs to do, 
etc.

I also increased the default value of the former from 1% to 5%. Charlie 
ran some performance tests and found that the new default eliminated 
some very long mixed GCs that he was experiencing with the old default.

Tony