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Details:
1- Stores info into proto format and writes to a file.
2- The cache is instantiated & owned by ChromeDownloadManagerDelegate.
3- The entry is created when download starts and will be deleted as
soon as the download completes.

This is continued from this CL by shaktisahu@:
https://chromium-review.googlesource.com/c/chromium/src/+/731403.

Bug: 778425
Change-Id: I6bd3f6df2648ac7b812c464b572492fb670d32bb
Reviewed-on: https://chromium-review.googlesource.com/753537
Commit-Queue: Joy Ming <jming@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516024}