Revert "Distinguish beween final incremental mark-compact and full mark-compact event in IdleNotification."

This reverts commit d15d453f for
breaking cctest/test-api/Threading3.

TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/758163003

Cr-Commit-Position: refs/heads/master@{#25506}