RFR: 8277865: G1: Change integer division to floating point division

[Leo Korinth]

In the class G1PreConcurrentStartTask::NoteStartOfMarkTask the method `worker_cost` returns a cost of type double, unfortunately the return value is calculated using an integer division:

`return _claimer.n_regions() / regions_per_thread;`

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Commit messages:
 - 8277865: G1: Change integer division to floating point division

Changes: https://git.openjdk.java.net/jdk/pull/6577/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6577&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277865
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6577.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6577/head:pull/6577

PR: https://git.openjdk.java.net/jdk/pull/6577