RFR: 8234502: Merge GenCollectedHeap and SerialHeap [v7]

[Lei Zaakjyu]

> 8234502: Merge GenCollectedHeap and SerialHeap

Lei Zaakjyu has updated the pull request incrementally with one additional commit since the last revision:

  fix import statement

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16927/files
  - new: https://git.openjdk.org/jdk/pull/16927/files/f6ca5177..6d8381cb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16927&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16927&range=05-06

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16927.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16927/head:pull/16927

PR: https://git.openjdk.org/jdk/pull/16927