RFR (S) 8223391: Unsynchronized iteration of ClassLoaderDataGraph

[coleen.phillimore at oracle.com]

Summary: lock in G1 before clear_claimed_marks

See bug for details:

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8223391.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8223391

Thanks,
Coleen

Ran|mach5 
hs-tier5-gc,hs-tier6-gc,hs-tier7-gc,hs-tier8-gc,hs-tier1,hs-tier2,hs-tier3||
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