Ensure both PanelBrowserWindowGtk and BalloonCollectionImpl call RemoveObserver.

chrome/browser/ui/panels/panel_browser_window_gtk.cc

 // Copyright (c) 2012 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 "chrome/browser/ui/panels/panel_browser_window_gtk.h"
 
 #include "base/bind.h"
 #include "chrome/browser/ui/browser_list.h"
 #include "chrome/browser/ui/gtk/browser_titlebar.h"
 #include "chrome/browser/ui/panels/panel.h"
@@ skipping 51 matching lines @@
       drag_end_factory_(this),
       panel_(panel),
       bounds_(bounds),
       window_size_known_(false),
       is_drawing_attention_(false),
       window_has_mouse_(false),
       show_close_button_(true) {
 }
 
 PanelBrowserWindowGtk::~PanelBrowserWindowGtk() {
+  CleanupDragDrop();
 }
 
 void PanelBrowserWindowGtk::Init() {
   BrowserWindowGtk::Init();
 
   // Keep the window always on top.
   gtk_window_set_keep_above(window(), TRUE);
 
   // Show the window on all the virtual desktops.
   gtk_window_stick(window());
@@ skipping 795 matching lines @@
     MessageLoopForUI::current()->RunAllPending();
 }
 
 bool NativePanelTestingGtk::IsWindowSizeKnown() const {
   return panel_browser_window_gtk_->window_size_known_;
 }
 
 bool NativePanelTestingGtk::IsAnimatingBounds() const {
   return panel_browser_window_gtk_->IsAnimatingBounds();
 }