RFR: 8279189: Inaccurate comment about class VMThread [v2]

[Xin Liu]

> The comment is inaccurate now. VMThread is different from the primordial thread. I guess the comment is based on the old codebase.
> Because VMThread is singleton, I put the Constructor and Destructor in "private" section. clients use static member functions create/destroy.

Xin Liu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into JDK-8279189
 - hide VMThread::loop as well.
 - 8279189: Inaccurate comment about class VMThread

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6922/files
  - new: https://git.openjdk.java.net/jdk/pull/6922/files/8c26760b..45c43219

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6922&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6922&range=00-01

  Stats: 9722 lines in 392 files changed: 5240 ins; 2974 del; 1508 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6922.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6922/head:pull/6922

PR: https://git.openjdk.java.net/jdk/pull/6922