RFR: JDK-8263495: Gather liveness info in the mark phase of G1 full gc

[Hamlin Li]

Gather liveness info in the mark phase of G1 full gc.

Per-region liveness info in the mark phase of G1 full gc is for several purposes:

JDK-8262068 needs it to determine whether compaction of a region should be skipped
JDK-8258431 for a JFR event that prints live set size estimate

so add this functionality.

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Commit messages:
 - JDK-8263495: Gather liveness info in the mark phase of G1 full gc

Changes: https://git.openjdk.java.net/jdk/pull/2966/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2966&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263495
  Stats: 59 lines in 10 files changed: 55 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2966.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2966/head:pull/2966

PR: https://git.openjdk.java.net/jdk/pull/2966