MD Downloads: add missing dependency in downloads.Item#computeDate_
MD Downloads: add missing dependency in downloads.Item#computeDate_

This manifested in missing dates if you:
1) delete the first download of a day
2) restore the delete
3) delete the same item again
4) restore again

R=groby@chromium.org
BUG=541770

Review URL: https://codereview.chromium.org/1399603004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353427}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk