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Currently we cache all the DownloadDBEntry when loading in-progress downloads.
However, this is not necessary and it wastes a lot of memory.
This CL no longer caches those entries, and will only cache them later
when OnDownloadUpdated() is called on an in-progress download.

BUG=893651

Change-Id: Id5c64f0efe6769446803d2b3ac4b58168917a27d
Reviewed-on: https://chromium-review.googlesource.com/c/1340950
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609552}