RFR: 8373335: Serial: Clean up SerialHeap members by access specifies [v2]

[Albert Mingkun Yang]

> Trivial moving member of `SerialHeap` so that all `private` fields/methods are altogether (at the top).
>  
> Test: tier1

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28718/files
  - new: https://git.openjdk.org/jdk/pull/28718/files/d3d84874..e12f7409

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28718&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28718&range=00-01

  Stats: 4 lines in 2 files changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28718/head:pull/28718

PR: https://git.openjdk.org/jdk/pull/28718