[Sync] Add CoreTypes support

sync/internal_api/public/base/model_type.h

Index: sync/internal_api/public/base/model_type.h
diff --git a/sync/internal_api/public/base/model_type.h b/sync/internal_api/public/base/model_type.h
index 59f970fd86f267bffc26d7dba5030b32148fb98b..820a6c74b58971315f29e9f6e7f22f42af83c0b9 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -207,6 +207,14 @@ SYNC_EXPORT ModelTypeSet ControlTypes();
 // See comment above for more information on what makes these types special.
 SYNC_EXPORT bool IsControlType(ModelType model_type);
 
+// Core types are those data types used by sync's core functionality (i.e. not
+// user data types), but which do not need to be treated as control
+// types (and do not need to be downloaded/associated before the backend
+// initializes).
+// These types are always enabled.
+SYNC_EXPORT ModelTypeSet PriorityCoreTypes();
+SYNC_EXPORT ModelTypeSet CoreTypes();

[tim (not reviewing), 2013/07/16 16:54:55]

I wonder if the "Core" distinction belongs in sync/ vs chrome/browser/sync...
fine for now, maybe Raghu can think about it.

+
 // Determine a model type from the field number of its associated
 // EntitySpecifics field.  Returns UNSPECIFIED if the field number is
 // not recognized.