Moves maintaining ShellObservers back to Shell

ash/shelf/shelf_widget_unittest.cc

Index: ash/shelf/shelf_widget_unittest.cc
diff --git a/ash/shelf/shelf_widget_unittest.cc b/ash/shelf/shelf_widget_unittest.cc
index 2e8941afa719b062ec71bc5f0d8cf8c545dc6899..de411877762d7d79b9926c0ca8e8dd19b6d1e682 100644
--- a/ash/shelf/shelf_widget_unittest.cc
+++ b/ash/shelf/shelf_widget_unittest.cc
@@ -331,9 +331,11 @@ class ShelfInitializer : public ShellObserver {
  public:
   ShelfInitializer(ShelfAlignment alignment, ShelfAutoHideBehavior auto_hide)
       : alignment_(alignment), auto_hide_(auto_hide) {
-    WmShell::Get()->AddShellObserver(this);
+    Shell::GetInstance()->AddShellObserver(this);
+  }
+  ~ShelfInitializer() override {
+    Shell::GetInstance()->RemoveShellObserver(this);
   }
-  ~ShelfInitializer() override { WmShell::Get()->RemoveShellObserver(this); }
 
   // ShellObserver:
   void OnShelfCreatedForRootWindow(WmWindow* root_window) override {