Remove linked_ptr from ash

ash/wm/video_detector.h

Index: ash/wm/video_detector.h
diff --git a/ash/wm/video_detector.h b/ash/wm/video_detector.h
index 5ccad543429e61bf5208b32ddb3bab921b0f5611..b3cbdec052c0b296affaf8c874955207735e13a4 100644
--- a/ash/wm/video_detector.h
+++ b/ash/wm/video_detector.h
@@ -13,7 +13,6 @@
 #include "ash/common/wm_window_observer.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
-#include "base/memory/linked_ptr.h"
 #include "base/observer_list.h"
 #include "base/scoped_observer.h"
 #include "base/time/time.h"
@@ -109,9 +108,6 @@ class ASH_EXPORT VideoDetector : public aura::EnvObserver,
   void OnWindowDestroying(WmWindow* window) override;
 
  private:
-  class WindowInfo;
-  typedef std::map<aura::Window*, linked_ptr<WindowInfo>> WindowInfoMap;
-
   // Called when video activity is observed in |window|.
   void HandleVideoActivity(aura::Window* window, base::TimeTicks now);
 
@@ -132,6 +128,8 @@ class ASH_EXPORT VideoDetector : public aura::EnvObserver,
   std::set<WmWindow*> fullscreen_root_windows_;
 
   // Maps from a window that we're tracking to information about it.
+  class WindowInfo;
+  using WindowInfoMap = std::map<aura::Window*, std::unique_ptr<WindowInfo>>;
   WindowInfoMap window_infos_;
 
   base::ObserverList<Observer> observers_;