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If the user pulls out the SD card, currently the download database
record will be removed and user can no longer see the download.

Instead, we shouldn't erase the database records for files on SD card
when checking if they are externally removed. Later we can add a timestamp
to persist the time when the file is found missing for the first time, and
erase the db record 30 days after this timestamp.

TBR=mpearson@chromium.org

Bug: 845369
Change-Id: I5b129df435119d501867cedde85ef49ec844aa38
Reviewed-on: https://chromium-review.googlesource.com/1068273
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561369}