RFR: 8366363: MemBaseline accesses VMT without using lock [v6]

[Johan Sjölen]

> Hi,
> 
> The `MemBaseline` used to access the VMT instance directly without a lock. We fix that, and we switch from using a `LinkedList` to a copied `RegionsTree` instead.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  It's const

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27003/files
  - new: https://git.openjdk.org/jdk/pull/27003/files/69c65fe1..7deb9a1b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27003.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27003/head:pull/27003

PR: https://git.openjdk.org/jdk/pull/27003