First shot at implementing drag&drop for Aura

ui/aura/client/window_drag_drop_delegate.h

Index: ui/aura/client/window_drag_drop_delegate.h
diff --git a/ui/aura/client/window_drag_drop_delegate.h b/ui/aura/client/window_drag_drop_delegate.h
new file mode 100644
index 0000000000000000000000000000000000000000..866035b5804da3f96c8962f445891b966481c62c
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+++ b/ui/aura/client/window_drag_drop_delegate.h
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+// 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_AURA_CLIENT_WINDOW_DRAG_DROP_DELEGATE_H_
+#define UI_AURA_CLIENT_WINDOW_DRAG_DROP_DELEGATE_H_
+#pragma once
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+
+class DropTargetEvent;
+
+// Delegate interface for drag and drop actions on aura::Window.
+class AURA_EXPORT WindowDragDropDelegate {
+ public:
+  // A window that supports drag and drop must override this and return true if
+  // data contains a type that may be dropped on this window.
+  virtual bool CanDrop(const DropTargetEvent& event) = 0;
+
+  // OnDragEntered is invoked when the mouse enters this window during a drag &
+  // drop session and CanDrop returns true. This is immediately
+  // followed by an invocation of OnDragUpdated, and eventually one of
+  // OnDragExited or OnPerformDrop.
+  virtual void OnDragEntered(const DropTargetEvent& event) = 0;
+
+  // Invoked during a drag and drop session while the mouse is over the window.
+  // This should return a bitmask of the DragDropTypes::DragOperation supported
+  // based on the location of the event. Return 0 to indicate the drop should
+  // not be accepted.
+  virtual int OnDragUpdated(const DropTargetEvent& event) = 0;
+
+  // Invoked during a drag and drop session when the mouse exits the window, or
+  // when the drag session was canceled and the mouse was over the window.
+  virtual void OnDragExited() = 0;
+
+  // Invoked during a drag and drop session when OnDragUpdated returns a valid
+  // operation and the user release the mouse.
+  virtual int OnPerformDrop(const DropTargetEvent& event) = 0;
+
+ protected:
+  virtual ~WindowDragDropDelegate() {}
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_CLIENT_WINDOW_DRAG_DROP_DELEGATE_H_