RFR: 8299853: Serial: Use more concrete type for TenuredGeneration::_the_space [v2]

[Albert Mingkun Yang]

> Simple change of using the actual type for a field.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11925/files
  - new: https://git.openjdk.org/jdk/pull/11925/files/f2356ce3..3b485fce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11925&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11925&range=00-01

  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11925.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11925/head:pull/11925

PR: https://git.openjdk.org/jdk/pull/11925