RFR: 8209976: Improve iteration over non-JavaThreads

[Roman Kennke]

Hi Kim,

may I ask what is the motivation for this? How does it improve the
situation?

Thanks,
Roman

> Please review this change to improve the iteration over
> non-JavaThreads.  This change introduces a new base class
> NonJavaThread, which should be a base for any Thread class that isn't
> a JavaThread.  The recently introduced iteration over NamedThreads
> (JDK-8209850) is moved to this new class.  This allows the
> re-implementation of Threads::non_java_threads_do using that list.
> 
> Note: Perhaps CollectedHeap::gc_threads_do could be replaced by a
> Threads::gc_threads_do that is implemented as an iteration over the
> non-JavaThreads with a filter to select GC-related thread types.  That
> isn't being done as part of this change though.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8209850
> 
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8209976/open.00/
> 
> Testing:
> mach5 tier1-3, hs-tier4-5 in conjunction with a fix for JDK-8209975.
> Local testing of just this change.
>