Fix crash in the experimental app list StartPageView.

ui/app_list/views/start_page_view.cc

 // Copyright 2014 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/app_list/views/start_page_view.h"
 
 #include "base/strings/utf_string_conversions.h"
 #include "content/public/browser/web_contents.h"
 #include "ui/app_list/app_list_constants.h"
 #include "ui/app_list/app_list_item.h"
@@ skipping 103 matching lines @@
     tiles_container_->AddChildView(tile_item);
     tile_views_.push_back(tile_item);
   }
 
   SetModel(view_delegate_->GetModel());
   view_delegate_->AddObserver(this);
 }
 
 StartPageView::~StartPageView() {
   view_delegate_->RemoveObserver(this);
+  if (model_)
+    model_->RemoveObserver(this);
 }
 
 void StartPageView::SetModel(AppListModel* model) {
   DCHECK(model);
   if (model_)
     model_->RemoveObserver(this);
   model_ = model;
   model_->AddObserver(this);
   Reset();
 }
@@ skipping 33 matching lines @@
 
 void StartPageView::OnAppListItemDeleted() {
   Reset();
 }
 
 void StartPageView::OnAppListItemUpdated(AppListItem* item) {
   Reset();
 }
 
 }  // namespace app_list