RFR(xs): 8198271: Make CollectedHeap::create_heap_space_summary() virtual

[Kim Barrett]

> On Feb 16, 2018, at 8:56 AM, Per Liden <per.liden at oracle.com> wrote:
> 
> This patch makes CollectedHeap::create_heap_space_summary() virtual, to allow a GC to override it. The current default implementation assumes that the "heap space" is the same as the "reserved_region", which isn't necessarily true. ZGC (and possibly Shenandoah in the future?) have a different heap layout and thus needs to override this function.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8198271
> Webrev: http://cr.openjdk.java.net/~pliden/8198271/webrev.0/
> 
> /Per

Looks good, and trivial.