[vm, gc] Use a clear, decreasing-cost order in deciding idle GC actions.
[vm, gc] Use a clear, decreasing-cost order in deciding idle GC actions.

Change-Id: I873676bf7e7ba6d98505bbd2dad604a12ae56f0e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150168
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk