Enable async mode for DownloadURL drags.

ui/base/dragdrop/os_exchange_data_provider_win.cc

Index: ui/base/dragdrop/os_exchange_data_provider_win.cc
diff --git a/ui/base/dragdrop/os_exchange_data_provider_win.cc b/ui/base/dragdrop/os_exchange_data_provider_win.cc
index 3b614bc15618736b9376ba580eb160c20941be19..e1f1bc587ae047c0a4df5806dcdeda54d8877f21 100644
--- a/ui/base/dragdrop/os_exchange_data_provider_win.cc
+++ b/ui/base/dragdrop/os_exchange_data_provider_win.cc
@@ -531,6 +531,9 @@ void OSExchangeDataProviderWin::SetDownloadFileInfo(
       Clipboard::GetCFHDropFormatType().ToFormatEtc(), storage);
   info->downloader = download.downloader;
   data_->contents_.push_back(info);
+
+  // Adding a download file always enables async mode.
+  data_->SetAsyncMode(TRUE);

[scottmg, 2015/04/07 19:38:05]

it looks like it should be VARIANT_TRUE from
https://msdn.microsoft.com/en-us/library/windows/desktop/bb776311(v=vs.85).aspx

i don't otherwise have any insight. i would just try dragging 2 files one after
the other, so if you tried that, then lgtm.

[dcheng, 2015/04/07 21:04:00]

Fixed this and several other instances.

 }
 
 void OSExchangeDataProviderWin::SetDragImage(