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This CL adds image cache protobuf and store interfaces. No
implementation is added yet.

The raw bytes are saved in a separate store, which only supports loading
one image at a time in its API.

Bug: 1058534
Change-Id: I0646b9906b81432e39f29dd5f235481d12a756c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108180Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Hesen Zhang <hesen@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751729}