sync: Implementation of Priority Preferences.

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 ebf76653f2241a471d676df8f362387fdb74b810..92df11666f4a0ae33420df96889d16bfbceea620 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -90,6 +90,9 @@ enum ModelType {
   FAVICON_IMAGES,
   // Favicon tracking information.
   FAVICON_TRACKING,
+  // These preferences are synced before other user types and are never
+  // encrypted.
+  PRIORITY_PREFERENCES,
 
   // ---- Proxy types ----
   // Proxy types are excluded from the sync protocol, but are still considered
@@ -113,10 +116,7 @@ enum ModelType {
   DEVICE_INFO,
   // Flags to enable experimental features.
   EXPERIMENTS,
-  // These preferences are never encrypted so that they can be applied before
-  // the encryption system is fully initialized.
-  PRIORITY_PREFERENCES,
-  LAST_CONTROL_MODEL_TYPE = PRIORITY_PREFERENCES,
+  LAST_CONTROL_MODEL_TYPE = EXPERIMENTS,
 
   LAST_REAL_MODEL_TYPE = LAST_CONTROL_MODEL_TYPE,
 
@@ -179,6 +179,10 @@ SYNC_EXPORT bool IsUserSelectableType(ModelType model_type);
 // This is the subset of UserTypes() that can be encrypted.
 SYNC_EXPORT_PRIVATE ModelTypeSet EncryptableUserTypes();
 
+// This is the subset of UserTypes() that have priority over other types.  These
+// types are synced before other user types and are never encrypted.
+SYNC_EXPORT ModelTypeSet PriorityUserTypes();
+
 // Proxy types are placeholder types for handling implicitly enabling real
 // types. They do not exist at the server, and are simply used for
 // UI/Configuration logic.