RFR: 8297173: usageTicks and totalTicks should be volatile to ensure that different threads get the latest ticks

[Poison]

As the title says, add the volatile modifier.

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Commit messages:
 - 8297173: usageTicks and totalTicks should be volatile

Changes: https://git.openjdk.org/jdk/pull/11199/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11199&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297173
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11199.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11199/head:pull/11199

PR: https://git.openjdk.org/jdk/pull/11199