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In this change only attachment payload is stored, support for metadata
will be in separate change.
Also in_memory_attachment_store_unittest is moved into test_template.
This test is now used for both stores.

BUG=
R=maniscalco@chromium.org

Review URL: https://codereview.chromium.org/652723003

Cr-Commit-Position: refs/heads/master@{#300041}