GitLab

When AddOrReplaceEntry() posts a UpdateDownloadDB task, RemoveEntry()
could be called before the posted task to get a executed.
This will cause the DCHECK to fail.

BUG=872279

Change-Id: I90c19a66814b050ef4cded72ee35380790d16d4c
Reviewed-on: https://chromium-review.googlesource.com/1169979Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581964}