RFR: 8264718: Shenandoah: enable string deduplication during root scanning

[Zhengyu Gu]

Shenandoah used to scan roots at pauses, so it deliberately disables string deduplication during root scanning to avoid extra pause times.

Now, Shenandoah scans roots in concurrent phase, it is no longer a concern, we should enable it.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - Merge branch 'master' into enable_dedup_roots
 - init

Changes: https://git.openjdk.java.net/jdk/pull/3348/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3348&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264718
  Stats: 69 lines in 5 files changed: 40 ins; 25 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3348.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3348/head:pull/3348

PR: https://git.openjdk.java.net/jdk/pull/3348