[Sync] Make all ModelTypeStore write methods go through WriteBatch.

components/sync/model/model_type_store.h

Index: components/sync/model/model_type_store.h
diff --git a/components/sync/model/model_type_store.h b/components/sync/model/model_type_store.h
index 07d1082d8273869ab228c40ddb3f969fcc7f4f6a..02448ea0c0fb30d2c79a4e555b27167368dfd91c 100644
--- a/components/sync/model/model_type_store.h
+++ b/components/sync/model/model_type_store.h
@@ -70,12 +70,18 @@ class ModelTypeStore {
   class WriteBatch {
    public:
     // Creates a MetadataChangeList that will accumulate metadata changes and
-    // can later pass them to a WriteBatch via TransferChanges. Use this when
+    // can later be passed to a WriteBatch via TransferChanges. Use this when
     // you need a MetadataChangeList and do not have a WriteBatch in scope.
     static std::unique_ptr<MetadataChangeList> CreateMetadataChangeList();
 
     virtual ~WriteBatch();
 
+    // Write the given |value| for data with |id|.
+    void WriteData(const std::string& id, const std::string& value);
+
+    // Delete the record for data with |id|.
+    void DeleteData(const std::string& id);
+
     // Provides access to a MetadataChangeList that will pass its changes
     // directly into this WriteBatch. Use this when you need a
     // MetadataChangeList and already have a WriteBatch in scope.
@@ -164,7 +170,12 @@ class ModelTypeStore {
   virtual void CommitWriteBatch(std::unique_ptr<WriteBatch> write_batch,
                                 const CallbackWithResult& callback) = 0;
 
-  // Write operations.
+ protected:
+  friend class AccumulatingMetadataChangeList;
+  friend class ModelTypeStoreImplTest;
+  friend class PassthroughMetadataChangeList;
+
+  // Write operations; access via WriteBatch.
   virtual void WriteData(WriteBatch* write_batch,
                          const std::string& id,
                          const std::string& value) = 0;