RFR: 8329659: Serial: Extract allowed_dead_ratio from ContiguousSpace

[Guoxiong Li]

Hi all,

This patch moves the `allowed_dead_ratio` related content from `ContiguousSpace` to `DeadSpacer`.

The tests `make test-tier1_gc` passed locally. Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8329659

Changes: https://git.openjdk.org/jdk/pull/18661/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18661&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329659
  Stats: 28 lines in 3 files changed: 8 ins; 11 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/18661.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18661/head:pull/18661

PR: https://git.openjdk.org/jdk/pull/18661