RFR: 8277394: Remove the use of safepoint_workers in reference processor

[Albert Mingkun Yang]

Simple change of passing workers to the reference processor constructor.

Test: hotspot_gc

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Commit messages:
 - safepoint_workers

Changes: https://git.openjdk.java.net/jdk/pull/6453/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6453&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277394
  Stats: 22 lines in 5 files changed: 10 ins; 1 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6453.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6453/head:pull/6453

PR: https://git.openjdk.java.net/jdk/pull/6453