[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..59f87e96e74f5d52319bb8c8a16be0389762561f 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 +42,14 @@ 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 whenever the callback is run. This can be useful for tests that
+  // want to verify the RecordingModelTypeChangeProcessor was given data by the
+  // bridge they are testing.
+  static ModelTypeSyncBridge::ChangeProcessorFactory FactoryForBridgeTest(
+      RecordingModelTypeChangeProcessor** processor_address);
+
  private:
   std::multimap<std::string, std::unique_ptr<EntityData>> put_multimap_;
   std::set<std::string> delete_set_;