RFR: 8304711: Combine G1 root region abort and wait into a single method

[Thomas Schatzl]

Hi all,

  merge root region scan and abort calls into a single method as they are used together twice; this also helps with [JDK-8300770](https://bugs.openjdk.org/browse/JDK-8300770) later.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8304711

Changes: https://git.openjdk.org/jdk/pull/13145/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13145&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304711
  Stats: 16 lines in 3 files changed: 8 ins; 2 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13145.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13145/head:pull/13145

PR: https://git.openjdk.org/jdk/pull/13145