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

chrome/browser/extensions/external_registry_loader_win.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTERNAL_REGISTRY_LOADER_WIN_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTERNAL_REGISTRY_LOADER_WIN_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/win/registry.h"
 #include "chrome/browser/extensions/external_loader.h"
 
 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_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalRegistryLoader);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_REGISTRY_LOADER_WIN_H_