Observe adding external extensions via windows registry.

chrome/browser/extensions/external_loader.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_LOADER_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTERNAL_LOADER_H_
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
@@ skipping 40 matching lines @@
   // By default, return an empty path, which indicates that relative paths
   // are not allowed.
   virtual const base::FilePath GetBaseCrxFilePath();
 
  protected:
   virtual ~ExternalLoader();
 
   // Notifies the provider that the list of extensions has been loaded.
   virtual void LoadFinished();
 
+  // Notifies the provider that the list of extensions has been updated.
+  virtual void OnUpdated(scoped_ptr<base::DictionaryValue> updated_prefs);

[lazyboy, 2016/01/26 23:44:43]

@asargent, as we discussed offline, I've made the change to
pass around the pref value while we are updating. Please take
a look at the diff between patch set #17 and #18

+
   // Used for passing the list of extensions from the method that loads them
   // to |LoadFinished|. To ensure thread safety, the rules are the following:
   // if this value is written on another thread than the UI, then it should
   // only be written in a task that was posted from |StartLoading|. After that,
   // this task should invoke |LoadFinished| with a PostTask. This scheme of
   // posting tasks will avoid concurrent access and imply the necessary memory
   // barriers.
   scoped_ptr<base::DictionaryValue> prefs_;
 
  private:
   friend class base::RefCountedThreadSafe<ExternalLoader>;
 
   ExternalProviderImpl* owner_;  // weak
 
   DISALLOW_COPY_AND_ASSIGN(ExternalLoader);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_LOADER_H_