RFR: 8265669: AccumCell should not be visible

[Johan Vos]

Hide accumCell when the cell is released.
Fix for JDK-8265669

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Commit messages:
 - Hide accumCell when the cell is released.

Changes: https://git.openjdk.java.net/jfx/pull/474/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=474&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265669
  Stats: 13 lines in 2 files changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/474.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/474/head:pull/474

PR: https://git.openjdk.java.net/jfx/pull/474