RFR: 8300053: Shenandoah: Handle more GCCauses in ShenandoahControlThread::request_gc

[Aleksey Shipilev]

$ CONF=linux-x86_64-server-fastdebug make test TEST=jdk/internal/vm/Continuation/BasicExt.java TEST_VM_OPTS="-XX:+UseShenandoahGC"

# Internal Error (/home/shade/trunks/jdk/src/hotspot/share/gc/shenandoah/shenandoahControlThread.cpp:479), pid=406430, tid=406562
# assert(GCCause::is_user_requested_gc(cause) || GCCause::is_serviceability_requested_gc(cause) || cause == GCCause::_metadata_GC_clear_soft_refs || cause == GCCause::_codecache_GC_aggressive || cause == GCCause::_codecache_GC_threshold || cause == GCCause::_full_gc_alot || cause == GCCause::_wb_full_gc || cause == GCCause::_wb_breakpoint || cause == GCCause::_scavenge_alot) failed: only requested GCs here: WhiteBox Initiated Young GC
#
``` 

Added a missing cause into the assert. The test starts to pass.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/11970/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11970&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300053
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11970.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11970/head:pull/11970

PR: https://git.openjdk.org/jdk/pull/11970