[jdk25u] RFR: 8369656: Calling CompletableFuture.join() could execute task in common pool

[Viktor Klang]

Backports the fix to https://bugs.openjdk.org/browse/JDK-8369656 from `master`.

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

Commit messages:
 - Backport b5b83247da9caea30c88b69543e350783663bc46

Changes: https://git.openjdk.org/jdk25u/pull/317/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=317&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369656
  Stats: 47 lines in 2 files changed: 43 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk25u/pull/317.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/317/head:pull/317

PR: https://git.openjdk.org/jdk25u/pull/317