Fix chrome.downloads.getFileIcon for hi-dpi and clarify docs.

chrome/browser/download/download_file_icon_extractor.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_DOWNLOAD_DOWNLOAD_FILE_ICON_EXTRACTOR_H_
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_FILE_ICON_EXTRACTOR_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "chrome/browser/icon_loader.h"
 
 // Helper class for DownloadsGetFileIconFunction. Only used for a single icon
 // extraction.
 class DownloadFileIconExtractor {
  public:
   // Callback for |ExtractIconForPath|. The parameter is a URL as a string for a
   // suitable icon. The string could be empty if the icon could not be
   // determined.
   typedef base::Callback<void(const std::string&)> IconURLCallback;
 
   virtual ~DownloadFileIconExtractor() {}
 
   // Should return false if the request was invalid.  If the return value is
   // true, then |callback| should be called with the result.
   virtual bool ExtractIconURLForPath(const base::FilePath& path,
+                                     float scale,
                                      IconLoader::IconSize icon_size,
                                      IconURLCallback callback) = 0;
 };
 
 #endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_FILE_ICON_EXTRACTOR_H_