10 RFR: 8169517: WhiteBox should provide concurrent GC phase control

[Kim Barrett]

Looking for reviewers.

> On Feb 17, 2017, at 8:44 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Please review this addition to the WhiteBox testing infrastructure,
> providing an API for putting a concurrent collector into a particular
> concurrent phase.
> 
> This is for writing tests where some action of the test needs
> to be performed while the collector is in a specific state. For
> example, to test a fix for JDK-8166188 we will need to dereference a
> jweak while G1 is doing concurrent marking. It is difficult to
> reliably arrange for that to happen without direct controls like those
> being provided here.
> 
> Only G1 support for the new feature is provided by this change. CMS
> support is TBD, and will be the subject of a new RFE.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8169517
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8169517/hs.00/
> http://cr.openjdk.java.net/~kbarrett/8169517/hotspot.00/
> 
> Testing:
> Change includes jtreg tests for the new feature.
> Local and jprt-based jtreg testing.