Add flag to enable immersive fullscreen for v2 apps

ash/wm/window_state_delegate.cc

 // Copyright 2013 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 "ash/wm/window_state_delegate.h"
 
 namespace ash {
 namespace wm {
 
 WindowStateDelegate::WindowStateDelegate() {
 }
 
 WindowStateDelegate::~WindowStateDelegate() {
 }
 
-bool WindowStateDelegate::ToggleFullscreen(WindowState* window_state) {
+bool WindowStateDelegate::EnterImmersiveFullscreen(WindowState* window_state) {
   return false;
 }
 
+bool WindowStateDelegate::ExitFullscreen(WindowState* window_state) {
+  return false;
+}
+
 }  // namespace wm
 }  // namespace ash