RFR (S): 8219369: Add named constants for iterating ExtRootScan phases

[Kim Barrett]

> On Feb 20, 2019, at 5:15 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi all,
> 
>  can I have reviews for this change that has been suggested as part of
> another review to improve readability of the ExtRootScan sub-phase
> printing?
> 
> In particular, it introduces a few new named constants in the
> GCParPhases enum that indicate start and end of the ext root scan sub-
> phases. These are then used in the iteration.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8219369
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8219369/webrev/
> testing:
> hs-tier1,hs-tier2 (includes log output testing for completeness)
> 
> Thanks,
>  Thomas

Looks good, but...

I have something of a preference against cluttering enums with such
pseudo-enumerators, and instead prefer class constants.  But I'll
leave it up to you.