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Datatype is responsible for writing attachments to AttachmentStore.
GenericChangeProcessor doesn't need attachment data in SyncData, only
list of attachment ids.

This change removes AttachmentList from SyncData and fixes corresponding
dependencies.

BUG=
R=maniscalco@chromium.org

Review URL: https://codereview.chromium.org/567053002

Cr-Commit-Position: refs/heads/master@{#294879}