RFR: 8262087: Use atomic boolean type in G1FullGCAdjustTask

[Albert Mingkun Yang]

Use atomic boolean type to make the intention clear.

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Commit messages:
 - atomic_bool

Changes: https://git.openjdk.java.net/jdk/pull/2664/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2664&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262087
  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2664.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2664/head:pull/2664

PR: https://git.openjdk.java.net/jdk/pull/2664