Remove Downcasting RVContextMenuBase in ContextMenuDeleagate implementations.

components/renderer_context_menu/render_view_context_menu_base.h

Index: components/renderer_context_menu/render_view_context_menu_base.h
diff --git a/components/renderer_context_menu/render_view_context_menu_base.h b/components/renderer_context_menu/render_view_context_menu_base.h
index 45b3fc54b37044a0c9c6260dc88d6f3ebcff40e5..f84cf39072e62c2e7ce0dcf59d13803e5f9c37fd 100644
--- a/components/renderer_context_menu/render_view_context_menu_base.h
+++ b/components/renderer_context_menu/render_view_context_menu_base.h
@@ -69,6 +69,10 @@ class RenderViewContextMenuBase : public ui::SimpleMenuModel::Delegate,
 
   ~RenderViewContextMenuBase() override;
 
+  // Displays the menu.
+  // Different platform will have their own implementation.
+  virtual void Show() = 0;
+
   // Initializes the context menu.
   void Init();