Ensure that FileSelectHelper is destroyed on the UI thread.

chrome/browser/file_select_helper.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_FILE_SELECT_HELPER_H_
 #define CHROME_BROWSER_FILE_SELECT_HELPER_H_
 
 #include <map>
 #include <vector>
 
 #include "base/compiler_specific.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "build/build_config.h"
+#include "content/public/browser/browser_thread.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/common/file_chooser_params.h"
 #include "net/base/directory_lister.h"
 #include "ui/shell_dialogs/select_file_dialog.h"
 
 #if defined(FULL_SAFE_BROWSING)
 #include "chrome/browser/safe_browsing/unverified_download_policy.h"
 #endif
 
 class Profile;
 
 namespace content {
 struct FileChooserFileInfo;
 class RenderViewHost;
 class WebContents;
 }
 
 namespace ui {
 struct SelectedFileInfo;
 }
 
 // This class handles file-selection requests coming from WebUI elements
 // (via the extensions::ExtensionHost class). It implements both the
 // initialisation and listener functions for file-selection dialogs.
-class FileSelectHelper : public base::RefCountedThreadSafe<FileSelectHelper>,
+class FileSelectHelper : public base::RefCountedThreadSafe<
+                             FileSelectHelper,
+                             content::BrowserThread::DeleteOnUIThread>,

[sky, 2016/01/21 18:06:27]

Please add a comment as to why this needs to be destroyed on the ui thread.

                          public ui::SelectFileDialog::Listener,
                          public content::WebContentsObserver,
                          public content::NotificationObserver {
  public:
-
   // Show the file chooser dialog.
   static void RunFileChooser(content::WebContents* tab,
                              const content::FileChooserParams& params);
 
   // Enumerates all the files in directory.
   static void EnumerateDirectory(content::WebContents* tab,
                                  int request_id,
                                  const base::FilePath& path);
 
  private:
   friend class base::RefCountedThreadSafe<FileSelectHelper>;
+  friend class base::DeleteHelper<FileSelectHelper>;
+  friend struct content::BrowserThread::DeleteOnThread<
+      content::BrowserThread::UI>;
+
   FRIEND_TEST_ALL_PREFIXES(FileSelectHelperTest, IsAcceptTypeValid);
   FRIEND_TEST_ALL_PREFIXES(FileSelectHelperTest, ZipPackage);
   FRIEND_TEST_ALL_PREFIXES(FileSelectHelperTest, GetSanitizedFileName);
   explicit FileSelectHelper(Profile* profile);
   ~FileSelectHelper() override;
 
   // Utility class which can listen for directory lister events and relay
   // them to the main object with the correct tracking id.
   class DirectoryListerDispatchDelegate
       : public net::DirectoryLister::DirectoryListerDelegate {
@@ skipping 167 matching lines @@
   content::NotificationRegistrar notification_registrar_;
 
   // Temporary files only used on OSX. This class is responsible for deleting
   // these files when they are no longer needed.
   std::vector<base::FilePath> temporary_files_;
 
   DISALLOW_COPY_AND_ASSIGN(FileSelectHelper);
 };
 
 #endif  // CHROME_BROWSER_FILE_SELECT_HELPER_H_