RFR: 8283715: Update ObjectStreamClass to be final

[Stuart Marks]

Pretty much just what it says.

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Commit messages:
 - Make ObjectStreamClass final.

Changes: https://git.openjdk.java.net/jdk/pull/8009/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8009&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283715
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8009.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8009/head:pull/8009

PR: https://git.openjdk.java.net/jdk/pull/8009