Reland "[heap] Start sweeper tasks after evacuation. (patchset #2 id:20001 of https://chromiumcodereview.appspot.com/2428043002/ )"

The performance regression in crbug.com/657776 was not caused by this CL.

This reverts commit 4490a760.

BUG=

Review-Url: https://chromiumcodereview.appspot.com/2437873004
Cr-Commit-Position: refs/heads/master@{#40478}