Pass shelf instances, not root windows, for ash context menus.

ash/test/test_shell_delegate.cc

Index: ash/test/test_shell_delegate.cc
diff --git a/ash/test/test_shell_delegate.cc b/ash/test/test_shell_delegate.cc
index b5d7fc0d1cb90da0bfd1887781020ef62d76d0a5..c0bbcbd23d4ffe5f2e64cba789df2198945538aa 100644
--- a/ash/test/test_shell_delegate.cc
+++ b/ash/test/test_shell_delegate.cc
@@ -175,7 +175,7 @@ MediaDelegate* TestShellDelegate::CreateMediaDelegate() {
 }
 
 ui::MenuModel* TestShellDelegate::CreateContextMenu(
-    aura::Window* root_window,
+    ash::Shelf* shelf,
     const ash::ShelfItem* item) {
   return nullptr;
 }