Enable async mode for DownloadURL drags.

ui/base/dragdrop/os_exchange_data_provider_win.cc

 // 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.
 
 #include "ui/base/dragdrop/os_exchange_data_provider_win.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
@@ skipping 513 matching lines @@
   // think we always synthesize one in WebContentsDragWin.
   STGMEDIUM* storage = NULL;
   if (!download.filename.empty())
     storage = GetStorageForFileName(download.filename);
 
   // Add CF_HDROP.
   DataObjectImpl::StoredDataInfo* info = new DataObjectImpl::StoredDataInfo(
       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(
     const gfx::ImageSkia& image,
     const gfx::Vector2d& cursor_offset) {
   drag_image_ = image;
   drag_image_offset_ = cursor_offset;
 }
 
 const gfx::ImageSkia& OSExchangeDataProviderWin::GetDragImage() const {
@@ skipping 503 matching lines @@
 
 ///////////////////////////////////////////////////////////////////////////////
 // OSExchangeData, public:
 
 // static
 OSExchangeData::Provider* OSExchangeData::CreateProvider() {
   return new OSExchangeDataProviderWin();
 }
 
 }  // namespace ui