RFR(XS): 8248799: Shenandoah: Claim threads token in constructor of ShenandoahRootVerifier

[rkennke at redhat.com]

In JDK-8248725 I changed thread iteration in ShenandoahRootVerifier
from serial to parallel, but we also need to change the thread-claim-
token in order for it to work. 

Bug: https://bugs.openjdk.java.net/browse/JDK-8248799
Testing: hotspot_gc_shenandoah

Fix:

diff -r 505a6165d85a -r 8c64644e2619
src/hotspot/share/gc/shenandoah/shenandoahRootVerifier.cpp
---
a/src/hotspot/share/gc/shenandoah/shenandoahRootVerifier.cpp        Fr
i Jul 03 11:09:59 2020 +0200
+++
b/src/hotspot/share/gc/shenandoah/shenandoahRootVerifier.cpp        Fri
Jul 03 14:31:11 2020 +0200
@@ -46,6 +46,7 @@
 STATIC_ASSERT((static_cast<uint>(ShenandoahRootVerifier::AllRoots) +
1) > static_cast<uint>(ShenandoahRootVerifier::AllRoots));
 
 ShenandoahRootVerifier::ShenandoahRootVerifier(RootTypes types) :
_types(types) {
+  Threads::change_thread_claim_token();
 }
 
 void ShenandoahRootVerifier::excludes(RootTypes types) {