Use shelf_view_ to get Shelf (via GetShelf())

Bug: None
Test: No functional change

Change-Id: I0ee6f733246f3e4473e8837842e66b67a7a3532b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703365
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690079}

ash/shelf/overflow_bubble_view.h

@@ -114,6 +114,9 @@ class ASH_EXPORT OverflowBubbleView : public ShelfBubble,
   // ui::EventHandler:
   void OnScrollEvent(ui::ScrollEvent* event) override;
 
+  Shelf* GetShelf();
+  const Shelf* GetShelf() const;
+
   mutable LayoutStrategy layout_strategy_;
 
   // Child views Owned by views hierarchy.
@@ -122,7 +125,6 @@ class ASH_EXPORT OverflowBubbleView : public ShelfBubble,
   OverflowShelfContainerView* shelf_container_view_ = nullptr;
 
   // Not owned.
-  Shelf* shelf_;
   ShelfView* shelf_view_;
 
   gfx::Vector2dF scroll_offset_;