[Sync] Support commit only types.

components/sync/model_impl/shared_model_type_processor.h

Index: components/sync/model_impl/shared_model_type_processor.h
diff --git a/components/sync/model_impl/shared_model_type_processor.h b/components/sync/model_impl/shared_model_type_processor.h
index 17ac9e9617057fda49f7b6e138a28306accb1efa..c64f8000d75415b29531d9a23383956238e95c2d 100644
--- a/components/sync/model_impl/shared_model_type_processor.h
+++ b/components/sync/model_impl/shared_model_type_processor.h
@@ -40,7 +40,8 @@ class SharedModelTypeProcessor : public ModelTypeProcessor,
  public:
   SharedModelTypeProcessor(ModelType type,
                            ModelTypeSyncBridge* bridge,
-                           const base::RepeatingClosure& dump_stack);
+                           const base::RepeatingClosure& dump_stack,
+                           bool commit_only);
   ~SharedModelTypeProcessor() override;
 
   // Whether the processor is allowing changes to its model type. If this is
@@ -218,6 +219,12 @@ class SharedModelTypeProcessor : public ModelTypeProcessor,
   // MergeSyncData/ApplySyncChanges.
   std::map<std::string, std::string> storage_key_to_tag_hash_;
 
+  // If the processor should behave as if |type_| is one of the commit only
+  // model types. For this processor, being commit only means that on commit
+  // confirmation, we should delete local data, because the model side never
+  // intends to read it. This includes both data and metadata.
+  bool commit_only_;
+
   SEQUENCE_CHECKER(sequence_checker_);
 
   // WeakPtrFactory for this processor which will be sent to sync thread.