RFR: JDK-8234502 : Merge GenCollectedHeap and SerialHeap [v7]

[Lei Zaakjyu]

> JDK-8234502 : Merge GenCollectedHeap and SerialHeap

Lei Zaakjyu has updated the pull request incrementally with three additional commits since the last revision:

 - replace a necessary include statement
 - clean up
 - add line-breaks

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16623/files
  - new: https://git.openjdk.org/jdk/pull/16623/files/1202a7bd..0563686a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16623&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16623&range=05-06

  Stats: 10 lines in 5 files changed: 1 ins; 6 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16623.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16623/head:pull/16623

PR: https://git.openjdk.org/jdk/pull/16623