RFR: 8278891: G1: Call reset in G1RegionMarkStatsCache constructor

[Hamlin Li]

Currently, G1RegionMarkStatsCache constructor does not call G1RegionMarkStatsCache::reset(), it depends on user to call it explicitly to make sure it works well and without crash in G1RegionMarkStatsCache::evict(uint idx).

In fact G1RegionMarkStatsCache constructor should call G1RegionMarkStatsCache::reset() to ensure its valid initial state.

( I met this issue when I tried to reuse this class to implement some parallel cache when improve the parallelism of evacuation failure. )

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6861/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6861&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278891
  Stats: 2 lines in 2 files changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6861.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6861/head:pull/6861

PR: https://git.openjdk.java.net/jdk/pull/6861