Revert of Reland concurrent sweeping of code space. (patchset #5 id:80001 of https://codereview.chromium.org/1225733002/)

Reason for revert:
Tests became flaky.

Original issue's description:
> Reland concurrent sweeping of code space.
>
> BUG=chromium:506778,chromium:506957,chromium:507211
> LOG=n
>
> Committed: https://crrev.com/806b81f11e3bfaef0d4330c7669e6934074be9cb
> Cr-Commit-Position: refs/heads/master@{#29748}

TBR=jochen@chromium.org,mvstanton@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:506778,chromium:506957,chromium:507211

Review URL: https://codereview.chromium.org/1242743002

Cr-Commit-Position: refs/heads/master@{#29751}