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The new leveldb manifest reuse feature was disabled because the CrOS dev channel
daily error % by channel values increased after this landed in that channel (by
about 3x). Reverting this change did not return the dev channel rates back to
normal. Furthermore the beta channel (with this feature enabled) did not
experience an increase in errors.

Commit details: Returns manifest reuse back to the state introduced in
crrev.com/803603004 (de701abe) which was disabled in two subsequent reverts:
crrev.com/841753003 (ca6c8754) and crrev.com/852073002 (617e94e4).

BUG=402980

Review URL: https://codereview.chromium.org/899933002

Cr-Commit-Position: refs/heads/master@{#314638}