mash: Use the new mus drag and drop API to get drag working in mash.

ui/views/widget/drop_helper.h

 // Copyright (c) 2011 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 UI_VIEWS_WIDGET_DROP_HELPER_H_
 #define UI_VIEWS_WIDGET_DROP_HELPER_H_
 
 #include "base/macros.h"
 
+#include "ui/views/views_export.h"
+
 namespace gfx {
 class Point;
 }  // namespace gfx
 
 namespace ui {
 class OSExchangeData;
 }  // namespace ui
 using ui::OSExchangeData;
 
 namespace views {
 
 class RootView;
 class View;
 
 // DropHelper provides support for managing the view a drop is going to occur
 // at during dnd as well as sending the view the appropriate dnd methods.
 // DropHelper is intended to be used by a class that interacts with the system
 // drag and drop. The system class invokes OnDragOver as the mouse moves,
 // then either OnDragExit or OnDrop when the drop is done.
-class DropHelper {
+class VIEWS_EXPORT DropHelper {
  public:
   explicit DropHelper(View* root_view);
   ~DropHelper();
 
   // Current view drop events are targeted at, may be NULL.
   View* target_view() const { return target_view_; }
 
   // Returns the RootView the DropHelper was created with.
   View* root_view() const { return root_view_; }
 
@@ skipping 58 matching lines @@
 
   // The deepest view under the current drop coordinate.
   View* deepest_view_;
 
   DISALLOW_COPY_AND_ASSIGN(DropHelper);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DROP_HELPER_H_