Run FileSelectHelper::ProcessSelectedFilesMac in parallel in TaskScheduler.

chrome/browser/file_select_helper_mac.mm

 // Copyright 2014 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.
 
 #include "chrome/browser/file_select_helper.h"
 
 #include <Cocoa/Cocoa.h>
 #include <sys/stat.h>
 
 #include "base/files/file.h"
@@ skipping 70 matching lines @@
 
   int result = -1;
   if (success)
     result = fchmod(file.GetPlatformFile(), S_IRUSR);
 
   return result >= 0 ? dest : base::FilePath();
 }
 
 void FileSelectHelper::ProcessSelectedFilesMac(
     const std::vector<ui::SelectedFileInfo>& files) {
-  DCHECK_CURRENTLY_ON(content::BrowserThread::FILE_USER_BLOCKING);
-
   // Make a mutable copy of the input files.
   std::vector<ui::SelectedFileInfo> files_out(files);
   std::vector<base::FilePath> temporary_files;
 
   for (auto& file_info : files_out) {
     NSString* filename = base::mac::FilePathToNSString(file_info.local_path);
     BOOL isPackage =
         [[NSWorkspace sharedWorkspace] isFilePackageAtPath:filename];
     if (isPackage && base::DirectoryExists(file_info.local_path)) {
       base::FilePath result = ZipPackage(file_info.local_path);
@@ skipping 30 matching lines @@
     // need to be deleted now.
     if (!web_contents_) {
       DeleteTemporaryFiles();
       RunFileChooserEnd();
       return;
     }
   }
 
   NotifyRenderFrameHostAndEnd(files);
 }