Reland "[heap] Fix a formatting bug in --trace-incremental-marking. (patchset #2 id:20001 of https://codereview.chromium.org/2324623004/ )"

This reverts commit 6c2a217e.

The performance regression in crbug.com/645411 was not caused
by the original CL.

TBR=mlippautz@chromium.org

Review-Url: https://codereview.chromium.org/2329983002
Cr-Commit-Position: refs/heads/master@{#39353}