Make UserActivityDetector a singleton

ash/shell.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_H_
 #define ASH_SHELL_H_
 
 #include <utility>
 #include <vector>
 
@@ skipping 350 matching lines @@
   }
   PowerButtonController* power_button_controller() {
     return power_button_controller_.get();
   }
   LockStateController* lock_state_controller() {
     return lock_state_controller_.get();
   }
   MruWindowTracker* mru_window_tracker() {
     return mru_window_tracker_.get();
   }
-  ::wm::UserActivityDetector* user_activity_detector() {
-    return user_activity_detector_.get();
-  }
   VideoDetector* video_detector() {
     return video_detector_.get();
   }
   WindowCycleController* window_cycle_controller() {
     return window_cycle_controller_.get();
   }
   WindowSelectorController* window_selector_controller() {
     return window_selector_controller_.get();
   }
   FocusCycler* focus_cycler() { return focus_cycler_.get(); }
@@ skipping 380 matching lines @@
 
   // Injected content::GPUDataManager support.
   scoped_ptr<GPUSupport> gpu_support_;
 
   DISALLOW_COPY_AND_ASSIGN(Shell);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SHELL_H_