Refine ash shelf tooltip closing on non-mash ChromeOS.

ash/shelf/shelf_widget.cc

Index: ash/shelf/shelf_widget.cc
diff --git a/ash/shelf/shelf_widget.cc b/ash/shelf/shelf_widget.cc
index 74d5abf858bc96784bd37872caddec206e1ed9fd..6458119a5e3fc0b13a6b1f63cd4b5ac8a305a77b 100644
--- a/ash/shelf/shelf_widget.cc
+++ b/ash/shelf/shelf_widget.cc
@@ -280,7 +280,10 @@ class ShelfWindowTargeter : public wm::EasyResizeWindowTargeter,
   }
 
   // ash::ShelfLayoutManagerObserver:
-  void WillDeleteShelf() override { shelf_ = NULL; }
+  void WillDeleteShelf() override {
+    shelf_->RemoveObserver(this);
+    shelf_ = NULL;
+  }
 
   void WillChangeVisibilityState(ash::ShelfVisibilityState new_state) override {
     gfx::Insets mouse_insets;