Fix windows registry loader to carry over prefs from file thread correctly

chrome/browser/extensions/external_registry_loader_win.h

Index: chrome/browser/extensions/external_registry_loader_win.h
diff --git a/chrome/browser/extensions/external_registry_loader_win.h b/chrome/browser/extensions/external_registry_loader_win.h
index 9b93cec1dbd54f6322450c5270efdc7b081f4fc7..f596d2d1a488c49e03a4645aee8d5002088e09a2 100644
--- a/chrome/browser/extensions/external_registry_loader_win.h
+++ b/chrome/browser/extensions/external_registry_loader_win.h
@@ -21,12 +21,15 @@ class ExternalRegistryLoader : public ExternalLoader {
 
   void StartLoading() override;
 
+  // Overridden to mock registry reading in unit tests.
+  virtual std::unique_ptr<base::DictionaryValue> LoadPrefsOnFileThread();
+
  private:
   friend class base::RefCountedThreadSafe<ExternalLoader>;
 
-  std::unique_ptr<base::DictionaryValue> LoadPrefsOnFileThread();
   void LoadOnFileThread();
-  void CompleteLoadAndStartWatchingRegistry();
+  void CompleteLoadAndStartWatchingRegistry(
+      std::unique_ptr<base::DictionaryValue> prefs);
   void UpdatePrefsOnFileThread();
   void OnRegistryKeyChanged(base::win::RegKey* key);