RFR: 8293544: G1: Add comment in G1BarrierSetC1::pre_barrier

[Albert Mingkun Yang]

A comment-only change in G1 pre-barrier to explain the redundant is-marking-active check.

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Commit messages:
 - g1-c1-comment

Changes: https://git.openjdk.org/jdk/pull/10219/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10219&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293544
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10219.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10219/head:pull/10219

PR: https://git.openjdk.org/jdk/pull/10219