RFR: 8171382: java.time.Duration missing isPositive method

[Naoto Sato]

Please review this PR to introduce `java.time.Duration.isPositive()` method. A CSR is also drafted.

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Commit messages:
 - 8171382: java.time.Duration missing isPositive method

Changes: https://git.openjdk.java.net/jdk/pull/4892/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4892&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8171382
  Stats: 31 lines in 2 files changed: 30 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4892.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4892/head:pull/4892

PR: https://git.openjdk.java.net/jdk/pull/4892