[Sync] Add support for proxy types

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 773cde549d6dc313fb9348109f8bbca1539eb4f7..8eaf5d0e51faa11a41bec0bb9746bf43c6e796dc 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -143,6 +143,11 @@ SYNC_EXPORT ModelType GetModelTypeFromSpecifics(
 // value (sibling ordering) for this item.
 bool ShouldMaintainPosition(ModelType model_type);
 
+// Protocol types are those types that have actual protocol buffer
+// representations. This distinguishes them from Proxy types, which have no
+// protocol representation and are never sent to the server.
+SYNC_EXPORT ModelTypeSet ProtocolTypes();
+
 // These are the user-selectable data types.  Note that some of these share a
 // preference flag, so not all of them are individually user-selectable.
 SYNC_EXPORT ModelTypeSet UserTypes();
@@ -150,6 +155,11 @@ SYNC_EXPORT ModelTypeSet UserTypes();
 // This is the subset of UserTypes() that can be encrypted.
 SYNC_EXPORT_PRIVATE ModelTypeSet EncryptableUserTypes();
 
+// 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.
+SYNC_EXPORT ModelTypeSet ProxyTypes();
+
 // Returns a list of all control types.
 //
 // The control types are intended to contain metadata nodes that are essential