RFR: 8264718: Shenandoah: enable string deduplication during root scanning [v2]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Removed excessive empty lines

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3348/files
  - new: https://git.openjdk.java.net/jdk/pull/3348/files/45344ee4..d3d63b38

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3348&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3348&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 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