RFR: 8364230: javax/swing/text/StringContent can be migrated away from using finalize

[Phil Race]

Convert StringContent's Position usage tracking to use a WeakReference

The test that is added only implicitly tests this but I added it because of a complete lack of any test for this code.

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Commit messages:
 - 8364230

Changes: https://git.openjdk.org/jdk/pull/26519/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26519&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364230
  Stats: 92 lines in 2 files changed: 71 ins; 14 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/26519.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26519/head:pull/26519

PR: https://git.openjdk.org/jdk/pull/26519