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This CL adds persistence for content data, and unittest for it.

Bug:831633

Change-Id: I944d9812133192e24306699bd696b5de8c2e411f
Reviewed-on: https://chromium-review.googlesource.com/1080274
Commit-Queue: Gang Wu <gangwu@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564775}