RFR: 8259488: Shenandoah: Missing timing tracking for STW CLD root processing

[Zhengyu Gu]

Please review this trivial patch that adds missing timing tracking for STW CLD root processing.

- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8259488

Changes: https://git.openjdk.java.net/jdk/pull/2165/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2165&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259488
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2165.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2165/head:pull/2165

PR: https://git.openjdk.java.net/jdk/pull/2165