[Sync] Have configuration specify which datatypes are ready to sync

[Sync] Have configuration specify which datatypes are ready to sync

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
Committed: https://crrev.com/92b21f8adf73d5198e05178b3bdc2ee5234c731f
Cr-Commit-Position: refs/heads/master@{#325582}

[Nicolas Zea, 2015-04-16 20:50:49]

zea@chromium.org changed reviewers:
+ pavely@chromium.org

[Nicolas Zea, 2015-04-16 20:50:50]

+Pavel, PTAL

[pavely, 2015-04-16 23:46:24]

lgtm

[Nicolas Zea, 2015-04-16 23:46:56]

The CQ bit was checked by zea@chromium.org

[commit-bot: I haz the power, 2015-04-16 23:50:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1081413003/60001

[commit-bot: I haz the power, 2015-04-17 02:00:23]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-04-17 02:01:22]

Patchset 4 (id:??) landed as
https://crrev.com/92b21f8adf73d5198e05178b3bdc2ee5234c731f
Cr-Commit-Position: refs/heads/master@{#325582}