Make drag and drop work on webpage. Also slightly modify drag drop workflow.

ui/aura_shell/drag_drop_controller.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_AURA_SHELL_DRAG_DROP_CONTROLLER_H_
 #define UI_AURA_SHELL_DRAG_DROP_CONTROLLER_H_
 #pragma once
 
 #include "ui/aura_shell/aura_shell_export.h"
 #include "ui/aura/client/drag_drop_client.h"
@@ skipping 21 matching lines @@
                                              public aura::EventFilter {
  public:
   DragDropController();
   virtual ~DragDropController();
 
   void set_should_block_during_drag_drop(bool should_block_during_drag_drop) {
     should_block_during_drag_drop_ = should_block_during_drag_drop;
   }
 
   // Overridden from aura::DragDropClient:
-  virtual void StartDragAndDrop(const ui::OSExchangeData& data,
+  virtual int StartDragAndDrop(const ui::OSExchangeData& data,
                                 int operation) OVERRIDE;
   virtual void DragUpdate(aura::Window* target,
                           const aura::MouseEvent& event) OVERRIDE;
   virtual void Drop(aura::Window* target,
                     const aura::MouseEvent& event) OVERRIDE;
   virtual void DragCancel() OVERRIDE;
+  virtual bool IsDragDropInProgress() OVERRIDE;
 
   // Overridden from aura::EventFilter:
   virtual bool PreHandleKeyEvent(aura::Window* target,
                                  aura::KeyEvent* event) OVERRIDE;
   virtual bool PreHandleMouseEvent(aura::Window* target,
                                    aura::MouseEvent* event) OVERRIDE;
   virtual ui::TouchStatus PreHandleTouchEvent(aura::Window* target,
                                               aura::TouchEvent* event) OVERRIDE;
 
  private:
@@ skipping 13 matching lines @@
   // Only be used for tests.
   bool should_block_during_drag_drop_;
 
   DISALLOW_COPY_AND_ASSIGN(DragDropController);
 };
 
 }  // namespace internal
 }  // namespace aura_shell
 
 #endif  // UI_AURA_SHELL_DRAG_DROP_CONTROLLER_H_