When the download folder does not exist, change the download folder to a user's "Downloads"

chrome/browser/ui/webui/options/advanced_options_handler.h

Index: chrome/browser/ui/webui/options/advanced_options_handler.h
diff --git a/chrome/browser/ui/webui/options/advanced_options_handler.h b/chrome/browser/ui/webui/options/advanced_options_handler.h
index 9a159563f9e9bc382c78b0617528f1f4cdb56993..3c54eb41183e3b743a6dc5d1ada6b5abbbc43cde 100644
--- a/chrome/browser/ui/webui/options/advanced_options_handler.h
+++ b/chrome/browser/ui/webui/options/advanced_options_handler.h
@@ -20,6 +20,37 @@ class AdvancedOptionsHandler
       public SelectFileDialog::Listener,
       public CloudPrintSetupHandlerDelegate {
  public:
+
+  class DownloadPathChecker
+      : public base::RefCountedThreadSafe<DownloadPathChecker> {
+   public:
+    explicit DownloadPathChecker(AdvancedOptionsHandler* handler);
+
+    // Check if the download folder still exists. If the download folder
+    // does not exist, the download folder is changed to the user's
+    // "Downloads" folder. This check runs in the background and may finish
+    // asynchronously after this method returns.
+    void CheckIfDownloadPathExists(const FilePath& path);
+
+   private:
+    friend class base::RefCountedThreadSafe<DownloadPathChecker>;
+    ~DownloadPathChecker();
+
+    // Called on the FILE thread to check the existence of the download folder.
+    // If it does not exist, this method tells the user's "Downloads"
+    // to OnDownloadPathChanged().
+    void CheckIfDownloadPathExistsOnFileThread(const FilePath& path);
+
+    // Called on the UI thread when the FILE thread finds that the download
+    // folder does not exist. This method changes the download folder to
+    // the user's "Downloads" folder and notifies this preference change
+    // to observers.
+    void OnDownloadPathChanged(const FilePath path);
+
+    // The handler we will report back to.
+    AdvancedOptionsHandler* handler_;
+  };
+
   AdvancedOptionsHandler();
   virtual ~AdvancedOptionsHandler();
 
@@ -132,7 +163,7 @@ class AdvancedOptionsHandler
 
   void SetupFontSizeLabel();
 
-  // Setup the download path based on user preferences.
+  // Setup the download folder based on user preferences.
   void SetupDownloadLocationPath();
 
   // Setup the pref whether to prompt for download location every time.
@@ -147,6 +178,8 @@ class AdvancedOptionsHandler
   // Setup the checked state for SSL related checkboxes.
   void SetupSSLConfigSettings();
 
+  scoped_refptr<DownloadPathChecker> download_path_checker_;
+
   scoped_refptr<SelectFileDialog> select_folder_dialog_;
 
 #if !defined(OS_CHROMEOS)