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Currently DownloadDB records some of the persistent for DownloadItem.
And DownloadItem will constantly requesting these data from the
DownloadDBCache during its life cycle. Which is unnecessary as these
data are already passed to DownloadItem during the creation time.
This CL allows DownloadItem to cache those persistent data after
creation. So there is no longer any need to request them again, and
we can remove them from the DownloadDBCache in future CLs to save
memory.

BUG=893651

Change-Id: Ic360bd81e283272105ef9d43d91e2b8dd00d9145
Reviewed-on: https://chromium-review.googlesource.com/c/1338328Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608949}