RFR: 8249877: Shenandoah: Report number of dead weak oops during STW weak roots

[Roman Kennke]

MemberNameLeak test is failing when running in STW mode, because
ResolvedMethodTable never receives a gc_notification() call, which is
usually done by calling report_num_dead() on the corresponding oop-
storage. We do that in concurrent-weak-processing, but apparently not
during STW weak processing. 

Bug:
https://bugs.openjdk.java.net/browse/JDK-8249877
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8249877/webrev.00/

Testing: hotspot_gc_shenandoah, MemberNameLeak.java, tier1 & tier2
without regressions.

Ok?