RFR: 8351276: Prevent redundant computeValue calls when a chain of mappings becomes observed

[John Hendrikx]

8351276: Prevent redundant computeValue calls when a chain of mappings becomes observed

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Commit messages:
 - Add test case
 - Prevent redundant compute value calls

Changes: https://git.openjdk.org/jfx/pull/1730/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1730&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351276
  Stats: 145 lines in 3 files changed: 138 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1730.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1730/head:pull/1730

PR: https://git.openjdk.org/jfx/pull/1730