The NextAction date has arrived: 2018-09-11

Cached image fetcher is considered nice to have for pre-stable experimenting (M71). We mark everything as P3 (not needed by the branch point). We'll re-prioritize after. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0b695ff80972cc1a65a5cd643186d2ae582cd4ac

commit 0b695ff80972cc1a65a5cd643186d2ae582cd4ac
Author: Brandon Wylie <wylieb@chromium.org>
Date: Thu Sep 13 18:01:09 2018

[IC] Implement leveldb metadata storage and unittests

Initial store for metadata, used for keeping track of which
images need to be garbage collected.

Bug:  878850 
Change-Id: I6d55c941c55ca0916196728761c2f9954759fb87
Reviewed-on: https://chromium-review.googlesource.com/1203414
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591071}
[modify] https://crrev.com/0b695ff80972cc1a65a5cd643186d2ae582cd4ac/components/image_fetcher/core/BUILD.gn
[modify] https://crrev.com/0b695ff80972cc1a65a5cd643186d2ae582cd4ac/components/image_fetcher/core/storage/BUILD.gn
[modify] https://crrev.com/0b695ff80972cc1a65a5cd643186d2ae582cd4ac/components/image_fetcher/core/storage/image_metadata_store.h
[modify] https://crrev.com/0b695ff80972cc1a65a5cd643186d2ae582cd4ac/components/image_fetcher/core/storage/image_metadata_store_leveldb.cc
[modify] https://crrev.com/0b695ff80972cc1a65a5cd643186d2ae582cd4ac/components/image_fetcher/core/storage/image_metadata_store_leveldb.h
[add] https://crrev.com/0b695ff80972cc1a65a5cd643186d2ae582cd4ac/components/image_fetcher/core/storage/image_metadata_store_leveldb_unittest.cc