Renames WmWindowUserData and converts to using aura

ash/wm/screen_pinning_controller.h

Index: ash/wm/screen_pinning_controller.h
diff --git a/ash/wm/screen_pinning_controller.h b/ash/wm/screen_pinning_controller.h
index 3245c60977d3230ec72eb13a33b39b4c18c81b53..a81afe1084e08a77d000221c6cf1978846da7a56 100644
--- a/ash/wm/screen_pinning_controller.h
+++ b/ash/wm/screen_pinning_controller.h
@@ -10,16 +10,13 @@
 
 #include "ash/display/window_tree_host_manager.h"
 #include "base/macros.h"
+#include "ui/aura/window_tracker.h"
 
 namespace ash {
 
-class WindowDimmer;
 class WindowTreeHostManager;
 class WmWindow;
 
-template <typename UserData>
-class WmWindowUserData;
-
 // Handles pinned state.
 class ScreenPinningController : public WindowTreeHostManager::Observer {
  public:
@@ -80,10 +77,10 @@ class ScreenPinningController : public WindowTreeHostManager::Observer {
   // Pinned window should be on top in the parent window.
   WmWindow* pinned_window_ = nullptr;
 
-  // Owns the WindowDimmers. There is one WindowDimmer for the parent of
-  // |pinned_window_| and one for each display other than the display
-  // |pinned_window_| is on.
-  std::unique_ptr<WmWindowUserData<WindowDimmer>> window_dimmers_;
+  // Maintains the set of aura::Windows that have a WindowDimmer installed on
+  // them. There is one WindowDimmer for the parent of |pinned_window_| and one
+  // for each display other than the display |pinned_window_| is on.
+  aura::WindowTracker windows_with_dimmers_;
 
   // Set true only when restacking done by this controller.
   bool in_restacking_ = false;