First shot at implementing drag&drop for Aura

ui/aura/window.cc

 // 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.
 
 #include "ui/aura/window.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "base/stl_util.h"
@@ skipping 12 matching lines @@
 
 namespace aura {
 
 Window::Window(WindowDelegate* delegate)
     : type_(WINDOW_TYPE_UNKNOWN),
       delegate_(delegate),
       parent_(NULL),
       transient_parent_(NULL),
       id_(-1),
       user_data_(NULL),
-      stops_event_propagation_(false) {
+      stops_event_propagation_(false),
+      drag_drop_delegate_(NULL) {
 }
 
 Window::~Window() {
+  // We need to NULL the drag drop delegate to prevent race condition where a
+  // drag/drop event may be sent to a window while its being destroyed.
+  drag_drop_delegate_ = NULL;
+
   // Let the delegate know we're in the processing of destroying.
   if (delegate_)
     delegate_->OnWindowDestroying();
 
   // Let the root know so that it can remove any references to us.
   Desktop* desktop = GetDesktop();
   if (desktop)
     desktop->WindowDestroying(this);
 
   // Then destroy the children.
@@ skipping 442 matching lines @@
 
   return delegate_ ? this : NULL;
 }
 
 void Window::OnPaintLayer(gfx::Canvas* canvas) {
   if (delegate_)
     delegate_->OnPaint(canvas);
 }
 
 }  // namespace aura