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Since we need to load multiple kinds of protos, this CL adds a generic
storage interface and its implementation to load protobuffers from
leveldb and converts them to in-memory structs. Actual store will be
subclass of the generic store class, and implement proto entry
conversions.

TBR=peter@chromium.org

Bug: 930968
Change-Id: I9b87ad14aeecd321ff2eccd4c11350354aae9b97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496527
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637899}