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To report download updates to extensions, chrome holds a json object in
memory so it can compare what has been changed since last update.
However, this json object is never released and it could cause memory
issues.
This CL removes the json object when download complete, and special
handle the case that the file is deleted after download completion.

Bug: 813055
Change-Id: I350ae6269f7ea27a47cf61dc3c38445190f35397
Reviewed-on: https://chromium-review.googlesource.com/982741Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551099}