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This CL adds impression store that talked to the level database that
persists impression types.

Bug: 930968
Change-Id: I6e1074901f78a8a4fc23fd33d77507426edc89fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560490
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649711}