Moves maintaining root_window_for_new_windows_ to Shell

ash/common/wm/window_positioning_utils.cc

 // Copyright 2016 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.
 
 #include "ash/common/wm/window_positioning_utils.h"
 
 #include <algorithm>
 
 #include "ash/common/wm/system_modal_container_layout_manager.h"
 #include "ash/common/wm/window_state.h"
@@ skipping 160 matching lines @@
       // the bounds if necessary.
       window->SetBounds(new_bounds);
 
       dst_container->AddChild(window);
 
       MoveAllTransientChildrenToNewRoot(display, window);
 
       // Restore focused/active window.
       if (focused && tracker.Contains(focused->aura_window())) {
         focused->SetFocused();
-        WmShell::Get()->set_root_window_for_new_windows(
+        Shell::GetInstance()->set_root_window_for_new_windows(
             focused->GetRootWindow());
       } else if (active && tracker.Contains(active->aura_window())) {
         active->Activate();
       }
       // TODO(oshima): We should not have to update the bounds again
       // below in theory, but we currently do need as there is a code
       // that assumes that the bounds will never be overridden by the
       // layout mananger. We should have more explicit control how
       // constraints are applied by the layout manager.
     }
   }
   gfx::Point origin(bounds_in_screen.origin());
   const gfx::Point display_origin =
       window->GetDisplayNearestWindow().bounds().origin();
   origin.Offset(-display_origin.x(), -display_origin.y());
   window->SetBounds(gfx::Rect(origin, bounds_in_screen.size()));
 }
 
 }  // namespace wm
 }  // namespace ash