RFR: 8290814: Hide G1RootRegions behind G1ConcurrentMark

[Thomas Schatzl]

Hi all,

  please review this cleanup that better encapsulates the `G1RootRegions` member of `G1ConcurrentMark` to be not directly accessible by non-concurrent mark related code. This is a preparatory change for [JDK-8289822](https://bugs.openjdk.org/browse/JDK-8289822) in that since the TAMSes are then owned by `G1ConcurrentMark`, there is the possibility to avoid allowing write access to these to non-G1ConcurrentMark classes as well.

[here](https://github.com/openjdk/jdk/compare/master...tschatzl:jdk:submit/8289822-move-tams?expand=1) you can see how that change could look like (also includes [JDK-8290357](https://bugs.openjdk.org/browse/JDK-8290357) that is also out for review).

Testing: tier1-3

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/9593/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9593&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290814
  Stats: 37 lines in 4 files changed: 24 ins; 11 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9593.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9593/head:pull/9593

PR: https://git.openjdk.org/jdk/pull/9593