RFR: 8267357: build breaks with -Werror option on micro benchmark added for JDK-8256973

[Jatin Bhateja]

Relevant declarations modified and tested with -Werror, no longer see unchecked conversion warnings.

Kindly review and approve.

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Commit messages:
 - 8267357: build breaks with -Werror option on micro benchmark added for JDK-8256973

Changes: https://git.openjdk.java.net/jdk/pull/4108/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4108&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267357
  Stats: 10 lines in 1 file changed: 0 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4108.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4108/head:pull/4108

PR: https://git.openjdk.java.net/jdk/pull/4108