views: Forward declare "class View;" in layout_manager.h

views/layout/layout_manager.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 "views/layout/layout_manager.h"
 
+#include "views/view.h"
+
 namespace views {
 
 LayoutManager::~LayoutManager() {
 }
 
 void LayoutManager::Installed(View* host) {
 }
 
 void LayoutManager::Uninstalled(View* host) {
 }
 
 int LayoutManager::GetPreferredHeightForWidth(View* host, int width) {
   return GetPreferredSize(host).height();
 }
 
 void LayoutManager::ViewAdded(View* host, View* view) {
 }
 
 void LayoutManager::ViewRemoved(View* host, View* view) {
 }
 
 }  // namespace views