Run FileSelectHelper::ProcessSelectedFilesMac in parallel in TaskScheduler.

chrome/browser/file_select_helper_mac.mm

Index: chrome/browser/file_select_helper_mac.mm
diff --git a/chrome/browser/file_select_helper_mac.mm b/chrome/browser/file_select_helper_mac.mm
index 525e5a0fbfce98263a5b7f4be0a376f871589ddf..2fdc57923d89d5f85f3039c61731c152fd150145 100644
--- a/chrome/browser/file_select_helper_mac.mm
+++ b/chrome/browser/file_select_helper_mac.mm
@@ -88,8 +88,6 @@ base::FilePath FileSelectHelper::ZipPackage(const base::FilePath& path) {
 
 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;