shelf: Allow dragging items from main shelf to overflow shelf.

ash/shelf/shelf_view.h

Index: ash/shelf/shelf_view.h
diff --git a/ash/shelf/shelf_view.h b/ash/shelf/shelf_view.h
index 894b62004b3b19e6246fbb6d34e48d10d4fa8a2a..498f43b93b4ec07b386e6696acbb377137c79314 100644
--- a/ash/shelf/shelf_view.h
+++ b/ash/shelf/shelf_view.h
@@ -230,6 +230,11 @@ class ASH_EXPORT ShelfView : public views::View,
   // Invoked when the mouse is dragged. Updates the models as appropriate.
   void ContinueDrag(const ui::LocatedEvent& event);
 
+  // Ends the drag on the other shelf. (ie if we are on main shelf, ends drag on
+  // the overflow shelf). Invoked when a shelf item is being dragged from one
+  // shelf to the other.
+  void EndDragOnOtherShelf(bool cancel);
+
   // Handles ripping off an item from the shelf. Returns true when the item got
   // removed.
   bool HandleRipOffDrag(const ui::LocatedEvent& event);
@@ -446,6 +451,9 @@ class ASH_EXPORT ShelfView : public views::View,
   // True when the icon was dragged off the shelf.
   bool dragged_off_shelf_ = false;
 
+  // True when an item is dragged from one shelf to another (eg. overflow).
+  bool dragged_to_another_shelf_ = false;
+
   // The rip off view when a snap back operation is underway.
   views::View* snap_back_from_rip_off_view_ = nullptr;