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In preparation for performing model association while other types are downloading,
the BackendDataTypeConfigurer::ConfigureDataTypes method will now return those
types that are ready to sync without further intervention from sync. These are the
types that were already syncing previously, and therefore don't need to be
downloaded or purged.

A followup patch will add the association logic and tests.

BUG=477063

Review URL: https://codereview.chromium.org/1081413003

Cr-Commit-Position: refs/heads/master@{#325582}