patch from issue 2577273002 at patchset 20001 (http://crrev.com/2577273002#ps20001)

chrome/browser/icon_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_ICON_LOADER_H_
 #define CHROME_BROWSER_ICON_LOADER_H_
 
 #include <memory>
 #include <string>
 
+#include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
-#include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_thread.h"
 #include "ui/gfx/image/image.h"
 
 ////////////////////////////////////////////////////////////////////////////////
 //
 // A facility to read a file containing an icon asynchronously in the IO
 // thread. Returns the icon in the form of an ImageSkia.
 //
 ////////////////////////////////////////////////////////////////////////////////
-class IconLoader : public base::RefCountedThreadSafe<IconLoader> {
+class IconLoader {
  public:
   // An IconGroup is a class of files that all share the same icon. For all
   // platforms but Windows, and for most files on Windows, it is the file type
   // (e.g. all .mp3 files share an icon, all .html files share an icon). On
   // Windows, for certain file types (.exe, .dll, etc), each file of that type
   // is assumed to have a unique icon. In that case, each of those files is a
   // group to itself.
   using IconGroup = base::FilePath::StringType;
 
   enum IconSize {
     SMALL = 0,  // 16x16
     NORMAL,     // 32x32
     LARGE,      // Windows: 32x32, Linux: 48x48, Mac: Unsupported
     ALL,        // All sizes available
   };
 
-  class Delegate {
-   public:
-    // Invoked when an icon has been read. |source| is the IconLoader. If the
-    // icon has been successfully loaded, |result| is non-null. |group| is the
-    // determined group from the original requested path.
-    virtual void OnImageLoaded(IconLoader* source,
-                               std::unique_ptr<gfx::Image> result,
-                               const IconGroup& group) = 0;
-
-   protected:
-    virtual ~Delegate() {}
-  };
+  // The callback invoked when an icon has been read. The parameters are:
+  // - The IconLoader.
+  // - The icon that was loaded, or null if there was a failure to load it.
+  // - The determined group from the original requested path.
+  using IconLoadedCallback = base::OnceCallback<
+      void(IconLoader*, std::unique_ptr<gfx::Image>, const IconGroup&)>;
 
   IconLoader(const base::FilePath& file_path,
              IconSize size,
-             Delegate* delegate);
+             IconLoadedCallback callback);
+  ~IconLoader();
 
   // Start reading the icon on the file thread.
   void Start();
 
  private:
-  friend class base::RefCountedThreadSafe<IconLoader>;
-
-  virtual ~IconLoader();
-
   // Given a file path, get the group for the given file.
   static IconGroup GroupForFilepath(const base::FilePath& file_path);
 
   // The thread ReadIcon() should be called on.
   static content::BrowserThread::ID ReadIconThreadID();
 
   void ReadGroup();
   void OnReadGroup();
   void ReadIcon();
 
-  void NotifyDelegate();
-
   // The task runner object of the thread in which we notify the delegate.
   scoped_refptr<base::SingleThreadTaskRunner> target_task_runner_;
 
   base::FilePath file_path_;
 
   IconGroup group_;
 
   IconSize icon_size_;
 
   std::unique_ptr<gfx::Image> image_;
 
-  Delegate* delegate_;
+  IconLoadedCallback callback_;
 
   DISALLOW_COPY_AND_ASSIGN(IconLoader);
 };
 
 #endif  // CHROME_BROWSER_ICON_LOADER_H_