[Ext Registry] Do not try to re-watch registry if we are already watching it.

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 e14fb864adcbb1b492f8a47d5107241f8257da95..9b93cec1dbd54f6322450c5270efdc7b081f4fc7 100644
--- a/chrome/browser/extensions/external_registry_loader_win.h
+++ b/chrome/browser/extensions/external_registry_loader_win.h
@@ -14,22 +14,25 @@ namespace extensions {
 
 class ExternalRegistryLoader : public ExternalLoader {
  public:
-  ExternalRegistryLoader() {}
+  ExternalRegistryLoader();
 
  protected:
+  ~ExternalRegistryLoader() override {}  // protected for unit test.
+
   void StartLoading() override;
 
  private:
   friend class base::RefCountedThreadSafe<ExternalLoader>;
 
-  ~ExternalRegistryLoader() override {}
-
   std::unique_ptr<base::DictionaryValue> LoadPrefsOnFileThread();
   void LoadOnFileThread();
   void CompleteLoadAndStartWatchingRegistry();
   void UpdatePrefsOnFileThread();
   void OnRegistryKeyChanged(base::win::RegKey* key);
 
+  // Whether or not we attempted to observe registry.
+  bool attempted_watching_registry_;
+
   base::win::RegKey hklm_key_;
   base::win::RegKey hkcu_key_;