RFR: Mark concurrent roots as global when requested to do so, instead of young

[Roman Kennke]

This is probably a merge error from the last big merge that brought in concurrent roots marking.

Testing:
 - [x] hotspot_gc_shenandoah
 - [ ] tier1 (+UseShenandoahGC)

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Commit messages:
 - Mark concurrent roots as global when requested to do so, instead of young

Changes: https://git.openjdk.java.net/shenandoah/pull/15/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=15&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/15.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/15/head:pull/15

PR: https://git.openjdk.java.net/shenandoah/pull/15