RFR: 8234502: Merge GenCollectedHeap and SerialHeap [v9]

[Lei Zaakjyu]

> 8234502: Merge GenCollectedHeap and SerialHeap

Lei Zaakjyu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:

 - resolve conflicts
 - Merge branch 'master' of https://git.openjdk.org/jdk into JDK-8234502
 - fix import statement
 - fix import statement
 - remove 'GenCollectedHeap' from 'jdk.hotspot.agent'
 - resolve conflict
 - Merge branch 'master' of https://git.openjdk.org/jdk into JDK-8234502
 - restore comment
 - line-break for EOF
 - merge 'CollectedHeap' and 'SerialHeap'

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Changes: https://git.openjdk.org/jdk/pull/16927/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16927&range=08
  Stats: 3170 lines in 21 files changed: 1507 ins; 1638 del; 25 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