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Two types of sync metadata, once stored in the sync directory, are now being
migrated to the web database. These two types are the per-entry EntityMetadata
and the ModelTypeState, which is globalstate for the autofill model type. This
change adds a migration to add the tables and functions on AutofillTable to
read/write/delete it, with associated tests.

BUG=671832

R=shess@chromium.org,mathp@chromium.org,pavely@chromium.org

Review-Url: https://codereview.chromium.org/2550293002
Cr-Commit-Position: refs/heads/master@{#437722}