Add persisted preference for projection touch HUD

ash/shell_observer.h

 // 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.
 
 #ifndef ASH_SHELL_OBSERVER_H_
 #define ASH_SHELL_OBSERVER_H_
 
 #include "ash/ash_export.h"
 #include "ash/system/user/login_status.h"
 
@@ skipping 14 matching lines @@
   // Invoked when the application is exiting.
   virtual void OnAppTerminating() {}
 
   // Invoked when the screen is locked (after the lock window is visible) or
   // unlocked.
   virtual void OnLockStateChanged(bool locked) {}
 
   // Invoked when the shelf alignment in |root_window| is changed.
   virtual void OnShelfAlignmentChanged(aura::RootWindow* root_window) {}
 
+  // Invoked when the projection touch HUD is toggled.
+  virtual void OnTouchHudProjectionToggled(bool enabled) {}
+
  protected:
   virtual ~ShellObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_OBSERVER_H_