mash: Add WmShell::SetDisplayWorkAreaInsets for ShelfLayoutManager.

ash/shelf/shelf_layout_manager.h

Index: ash/shelf/shelf_layout_manager.h
diff --git a/ash/shelf/shelf_layout_manager.h b/ash/shelf/shelf_layout_manager.h
index ab237b27655a8d16c61072f4e8ecc54a61a90e01..dfde44a197315e31da3a30f617a6a808050f26ba 100644
--- a/ash/shelf/shelf_layout_manager.h
+++ b/ash/shelf/shelf_layout_manager.h
@@ -301,11 +301,6 @@ class ASH_EXPORT ShelfLayoutManager
   void CompleteGestureDrag(const ui::GestureEvent& gesture);
   void CancelGestureDrag();
 
-  // The RootWindow is cached so that we don't invoke Shell::GetInstance() from
-  // our destructor. We avoid that as at the time we're deleted Shell is being
-  // deleted too.
-  aura::Window* root_window_;
-
   // True when inside UpdateBoundsAndOpacity() method. Used to prevent calling
   // UpdateBoundsAndOpacity() again from SetChildBounds().
   bool updating_bounds_;