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

ui/aura/client/window_drag_drop_delegate.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_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.
+  // drop session. 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_