G1: Use more accurate age in predict_eden_copy_time_ms

The arg of `G1SurvRateGroup::accum_surv_rate_pred` means the max age. In `G1Policy::predict_eden_copy_time_ms`, it should be called with `#eden-regions - 1` not `#eden-regions`, since age starts from 0.

(Using a larger value can result into more conservative estimates, e.g. more bytes to be evacuated during young-gc.)