RFR: 8285745: Re-examine PushbackInputStream mark/reset

[Brian Burkhalter]

Please review this request to remove the `synchronized` keyword from the `mark(int)` and `reset()` methods of `java.io.PushbackInputStream`.

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Commit messages:
 - 8285745: Re-examine PushbackInputStream mark/reset

Changes: https://git.openjdk.java.net/jdk/pull/8433/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8433&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285745
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8433.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8433/head:pull/8433

PR: https://git.openjdk.java.net/jdk/pull/8433