RFR: 8247402: rewrite the implementation requirements for Map::compute()

[John Lin]

This is from the mailing list: http://mail.openjdk.java.net/pipermail/core-libs-dev/2020-June/067213.html

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Commit messages:
 - 8247402: rewrite the implementation requirements for Map::compute()

Changes: https://git.openjdk.java.net/jdk/pull/451/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=451&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247402
  Stats: 10 lines in 1 file changed: 1 ins; 6 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/451.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/451/head:pull/451

PR: https://git.openjdk.java.net/jdk/pull/451