RFR: 8259774: samplethreads option does not work for jfr thread sampler [v2]

[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.

Yang Yi has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  8259774: samplethreads option  does not work for jfr thread sampler

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2079/files
  - new: https://git.openjdk.java.net/jdk/pull/2079/files/cfae38db..310f3d14

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2079&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2079&range=00-01

  Stats: 27 lines in 3 files changed: 21 ins; 4 del; 2 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