RFR: 8296124: G1: Call record_root_region_scan_wait_time conditionally

[Albert Mingkun Yang]

Simple change of making an update conditional in young-gc-pause.

Test: hotspot_gc

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Commit messages:
 - g1-conditional-record

Changes: https://git.openjdk.org/jdk/pull/10915/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10915&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296124
  Stats: 4 lines in 1 file changed: 1 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10915.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10915/head:pull/10915

PR: https://git.openjdk.org/jdk/pull/10915