RFR: 8278277 G1: Simplify implementation of G1GCPhaseTimes::record_or_add_time_secs

[Hamlin Li]

This is a minor improvement of G1GCPhaseTimes::record_or_add_time_secs to reuse WorkerDataArray<T>::set_or_add rather than implement the logic in G1GCPhaseTimes itself.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6715/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6715&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278277
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6715.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6715/head:pull/6715

PR: https://git.openjdk.java.net/jdk/pull/6715