RFR: 8271060: Merge G1CollectedHeap::determine_start_concurrent_mark_gc and G1Policy::decide_on_conc_mark_initiation

[Thomas Schatzl]

Hi all,

  can I have reviews for this merge of two methods that have the same purpose: determine whether this pause should be a concurrent start pause. The reason for this merge is that `G1CollectedHeap::determine_start_concurrent_mark_gc` just calls the other, and
  - is the only caller
  - only adds another condition of the same type that `G1Policy::decide_on_conc_mark_initiation` does at the beginning
  - adds some return value checking and return value
which all imho fit into `G1Policy::decide_on_conc_mark_initiation` too.

It also reduces `G1CollectedHeap` a bit. The main reason for me to put this in the new location is basically that that additional condition in  `G1CollectedHeap::determine_start_concurrent_mark_gc` makes all conditions for that decision be located closer together.

If you think otherwise and this is a bad idea, I will close this PR.

Testing: manual gc/g1 runs

Thanks,
  Thomas

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Commit messages:
 - Refactor decide-on-conc-mark-determination

Changes: https://git.openjdk.java.net/jdk/pull/4867/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4867&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271060
  Stats: 52 lines in 5 files changed: 22 ins; 20 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4867.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4867/head:pull/4867

PR: https://git.openjdk.java.net/jdk/pull/4867