RFR: 8276301: G1: Refine implementation of G1CardSetFreePool::memory_sizes()

[Hamlin Li]

Implementation of G1CardSetFreePool::memory_sizes() is not straight, this is to refine the implementation.

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Commit messages:
 - Add assert
 - Merge branch 'openjdk:master' into refine-G1CardSetFreePool-memory_sizes
 - Initial commits

Changes: https://git.openjdk.java.net/jdk/pull/6225/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6225&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276301
  Stats: 30 lines in 2 files changed: 6 ins; 22 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6225.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6225/head:pull/6225

PR: https://git.openjdk.java.net/jdk/pull/6225