Eliminate ash::Shell access in ShelfView

ash/aura/wm_shell_aura.h

Index: ash/aura/wm_shell_aura.h
diff --git a/ash/aura/wm_shell_aura.h b/ash/aura/wm_shell_aura.h
index 882afa59f31ff845f1a91fc2603ff2fa00837845..cd8b7b0291e140fa15ed114a698e8ca7b02ddd8b 100644
--- a/ash/aura/wm_shell_aura.h
+++ b/ash/aura/wm_shell_aura.h
@@ -45,6 +45,8 @@ class ASH_EXPORT WmShellAura : public WmShell,
   std::vector<WmWindow*> GetAllRootWindows() override;
   void RecordUserMetricsAction(UserMetricsAction action) override;
   void RecordTaskSwitchMetric(TaskSwitchSource source) override;
+  void ShowContextMenu(const gfx::Point& location_in_screen,
+                       ui::MenuSourceType source_type) override;
   std::unique_ptr<WindowResizer> CreateDragWindowResizer(
       std::unique_ptr<WindowResizer> next_window_resizer,
       wm::WindowState* window_state) override;