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This is disk caching part of Feed Image Loader, and other parts of Feed
Image Loader will come later.
leveldb_proto is used as database. This disk cache need to save images
when Image Loader fetched images from network, and next time when Image
Loader received a same URL request, disk cache can provided without
downloading. Also this disk chache will do timestamp-base garbage
collection when Image Loader requested.

Bug:807359, 807457

Change-Id: I66db2411304012cf6f04a7380fc34fb1f5306d2f
Reviewed-on: https://chromium-review.googlesource.com/981298
Commit-Queue: Gang Wu <gangwu@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548230}