RFR(S): 8227224: GenCollectedHeap: add subspace transitions for young gen for gc+heap=info log lines

[Kim Barrett]

> On Aug 7, 2019, at 10:05 AM, Tony Printezis <tprintezis at twitter.com> wrote:
> 
> Hi all,
> 
> Similar to 8227225 but for the GenCollectedHeap GCs. Webrev is here:
> 
> http://cr.openjdk.java.net/~tonyp/8227224/webrev.0/
> 
> Tony
> 
> 
> —————
> Tony Printezis | @TonyPrintezis | tprintezis at twitter.com

Looks good.

I had a somewhat longer comment about the similarities between this new
code for GenCollectedHeap and the recently added code for
ParallelScavengeHeap, but re-reading the previous thread I agree with
the plan to keep things relatively isolated for now and perhaps look for
possible refactorings once we have more complete coverage, particularly
including non-generational collectors.

One pre-existing nit I noticed while looking at the two side by side.
In ParallelScavengeHeap::get_pre_gc_values() the young->to_space()
value is fetched but not used.  I don't need a new webrev for removal
of that line.