Set DragDropTracker's capture window's size to zero, otherwise, it gets

ash/drag_drop/drag_drop_tracker.h

Index: ash/drag_drop/drag_drop_tracker.h
diff --git a/ash/drag_drop/drag_drop_tracker.h b/ash/drag_drop/drag_drop_tracker.h
index 220cc12a60241c938100fb63f26c9c419a65df5b..c16796758c793c04cbc4d8b4341d17d93b619ed3 100644
--- a/ash/drag_drop/drag_drop_tracker.h
+++ b/ash/drag_drop/drag_drop_tracker.h
@@ -11,7 +11,6 @@
 #include "ui/base/events/event.h"
 
 namespace aura {
-class RootWindow;
 class Window;
 }
 
@@ -25,7 +24,7 @@ namespace internal {
 // is supported for now.
 class ASH_EXPORT DragDropTracker {
  public:
-  explicit DragDropTracker(aura::RootWindow* root_window);
+  DragDropTracker();
   ~DragDropTracker();
 
   // Gets the target located at |event| in the coordinates of the root window