ui: Change ListModel to operate in size_t instead of int.

ui/base/models/list_model_observer.h

Index: ui/base/models/list_model_observer.h
diff --git a/ui/base/models/list_model_observer.h b/ui/base/models/list_model_observer.h
index 65b22a8f62cde7d77fcd0d6c5b6544a1d1735f97..49d40309ad8691f8c02f60e814cb1df8a0c1f363 100644
--- a/ui/base/models/list_model_observer.h
+++ b/ui/base/models/list_model_observer.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -6,6 +6,8 @@
 #define UI_BASE_MODELS_LIST_MODEL_OBSERVER_H_
 #pragma once
 
+#include <stddef.h>
+
 #include "ui/base/ui_export.h"
 
 namespace ui {
@@ -13,14 +15,14 @@ namespace ui {
 class UI_EXPORT ListModelObserver {
  public:
   // Invoked after items has been added to the model.
-  virtual void ListItemsAdded(int start, int count) = 0;
+  virtual void ListItemsAdded(size_t start, size_t count) = 0;
 
   // Invoked after items has been removed. |start| is the index before the
   // removal.
-  virtual void ListItemsRemoved(int start, int count) = 0;
+  virtual void ListItemsRemoved(size_t start, size_t count) = 0;
 
   // Invoked after items has been changed.
-  virtual void ListItemsChanged(int start, int count) = 0;
+  virtual void ListItemsChanged(size_t start, size_t count) = 0;
 
  protected:
   virtual ~ListModelObserver() {}