RFR: 8259774: samplethreads option does not work for jfr thread sampler

[Yang Yi]

Hi,

Please help review this minor change that lets the JFR option
`samplethreads` works.

In the current implementation, no matter whether `samplethreads`
option is turned on or off, it will not affect thread sampling. 
The sampler completely ignores it. This patch will address this 
problem and let the sampler aware of this option.

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Commit messages:
 - 8259774: samplethreads option  does not work for jfr thread sampler

Changes: https://git.openjdk.java.net/jdk/pull/2079/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2079&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259774
  Stats: 97 lines in 2 files changed: 96 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2079.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2079/head:pull/2079

PR: https://git.openjdk.java.net/jdk/pull/2079