[Sync] Create UserEventSyncBridge.

components/sync/model/recording_model_type_change_processor.h

Index: components/sync/model/recording_model_type_change_processor.h
diff --git a/components/sync/model/recording_model_type_change_processor.h b/components/sync/model/recording_model_type_change_processor.h
index 6fe9180166c8fd4fb35916eab8139cdbeb257b94..97260bead241e7db4d775ceea3889780224400d6 100644
--- a/components/sync/model/recording_model_type_change_processor.h
+++ b/components/sync/model/recording_model_type_change_processor.h
@@ -11,6 +11,7 @@
 #include <string>
 
 #include "components/sync/model/fake_model_type_change_processor.h"
+#include "components/sync/model/model_type_sync_bridge.h"
 
 namespace syncer {
 
@@ -41,6 +42,12 @@ class RecordingModelTypeChangeProcessor : public FakeModelTypeChangeProcessor {
 
   MetadataBatch* metadata() const { return metadata_.get(); }
 
+  // Returns a callback that constructs a processor and assigns a raw pointer to
+  // the given address. The caller must ensure that the address passed in is
+  // still valid when the callback is run.
+  static ModelTypeSyncBridge::ChangeProcessorFactory FactoryForBridgeTest(

[Patrick Noland, 2017/05/04 19:43:18]

Can you explain why it's necessary for the callback to both return and
side-assign the processor?

[skym, 2017/05/04 20:33:59]

Done.

+      RecordingModelTypeChangeProcessor** processor_address);
+
  private:
   std::multimap<std::string, std::unique_ptr<EntityData>> put_multimap_;
   std::set<std::string> delete_set_;