RFR: 8259404: Shenandoah: Fix time tracking in parallel_cleaning [v2]

[Zhengyu Gu]

> Please review this patch fixes timing tracking for parallel cleaning.
> 
> Before:
> `[9.844s][info][gc,stats] System Purge = 0.000 s (a = 76 us) (n = 1) (lvls, us = 76, 76, 76, 76, 76)` **<<== looks wrong**
> `[9.844s][info][gc,stats] Unload Classes = 0.001 s (a = 541 us) (n = 1) (lvls, us = 541, 541, 541, 541, 541)`
> `[9.844s][info][gc,stats] Weak Roots = 0.000 s (a = 75 us) (n = 1) (lvls, us = 75, 75, 75, 75, 75)`
> `[9.844s][info][gc,stats] CLDG = 0.000 s (a = 0 us) (n = 1) (lvls, us = 0, 0, 0, 0, 0)`
> After:
> `[9.936s][info][gc,stats] System Purge = 0.001 s (a = 611 us) (n = 1) (lvls, us = 609, 609, 609, 609, 611)`
> `[9.936s][info][gc,stats] Unload Classes = 0.000 s (a = 475 us) (n = 1) (lvls, us = 475, 475, 475, 475, 475)`
> `[9.936s][info][gc,stats] DCU: <total> = 0.000 s (a = 162 us) (n = 1) (lvls, us = 160, 160, 160, 160, 162)`
> `[9.936s][info][gc,stats] DCU: Code Cache Roots = 0.000 s (a = 162 us) (n = 1) (lvls, us = 160, 160, 160, 160, 162)`
> `[9.936s][info][gc,stats] Weak Roots = 0.000 s (a = 105 us) (n = 1) (lvls, us = 105, 105, 105, 105, 105)`
> `[9.936s][info][gc,stats] DWR: <total> = 0.000 s (a = 210 us) (n = 1) (lvls, us = 209, 209, 209, 209, 210)`
> `[9.936s][info][gc,stats] DWR: VM Weak Roots = 0.000 s (a = 210 us) (n = 1) (lvls, us = 209, 209, 209, 209)`

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

  Aleksey's comments

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2073/files
  - new: https://git.openjdk.java.net/jdk/pull/2073/files/3ae33066..33be84c9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2073&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2073&range=00-01

  Stats: 21 lines in 3 files changed: 6 ins; 4 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2073.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2073/head:pull/2073

PR: https://git.openjdk.java.net/jdk/pull/2073