[Sync] Add context proto and directory support

sync/syncable/directory.h

Index: sync/syncable/directory.h
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index ca8f4f133a72b3a2b95cfbde32688eda3e09d65e..5efa34836ec0e5e72afa5b4fb0425db9985eeb06 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -120,6 +120,8 @@ class SYNC_EXPORT Directory {
     // opaque to the client. This is the serialization of a message of type
     // ChipBag defined in sync.proto. It can contains NULL characters.
     std::string bag_of_chips;
+    // The per-datatype context.
+    sync_pb::DataTypeContext datatype_context[MODEL_TYPE_COUNT];
   };
 
   // What the Directory needs on initialization to create itself and its Kernel.
@@ -196,6 +198,14 @@ class SYNC_EXPORT Directory {
   int64 GetTransactionVersion(ModelType type) const;
   void IncrementTransactionVersion(ModelType type);
 
+  // Getter/setters for the per datatype context.
+  void GetDataTypeContext(BaseTransaction* trans,
+                          ModelType type,
+                          sync_pb::DataTypeContext* context) const;
+  void SetDataTypeContext(BaseWriteTransaction* trans,
+                          ModelType type,
+                          const sync_pb::DataTypeContext& context);
+
   ModelTypeSet InitialSyncEndedTypes();
   bool InitialSyncEndedForType(ModelType type);
   bool InitialSyncEndedForType(BaseTransaction* trans, ModelType type);