[11u] RFR (S) 8251451: Shenandoah: Remark ObjectSynchronizer roots with I-U

[Aleksey Shipilev]

Original bug:
   https://bugs.openjdk.java.net/browse/JDK-8251451
   https://cr.openjdk.java.net/~shade/8251451/webrev.11u.01/

The patch does apply cleanly to 11u, because the usual MutexLockerEx -> MutexLocker change, 
ShenandoahTaskTerminator -> TaskTerminator rename, indenting differences. Reapplied the hunks by hand.

11u webrev:
   https://cr.openjdk.java.net/~shade/8251451/webrev.11u.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}, tier{1,2} with Shenandoah

-- 
Thanks,
-Aleksey