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

ui/views/widget/drop_helper.h

Index: ui/views/widget/drop_helper.h
diff --git a/ui/views/widget/drop_helper.h b/ui/views/widget/drop_helper.h
index 4e740aafe3da669470c4245d983b768347a4410d..8c2c44fd69000ca6d45258163ca38b938da61f80 100644
--- a/ui/views/widget/drop_helper.h
+++ b/ui/views/widget/drop_helper.h
@@ -7,6 +7,8 @@
 
 #include "base/macros.h"
 
+#include "ui/views/views_export.h"
+
 namespace gfx {
 class Point;
 }  // namespace gfx
@@ -26,7 +28,7 @@ class View;
 // 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();