Ash: Ensure touch selection menus do not appear below the omnibox popup.

The touch menu is a regular popup bubble; using BubbleDialogDelegateView
to get the desired look with minimal effort. However, that engages its
CreateBubbleWidget(), which invokes widget->StackAbove(context). That
causes the bubble to stack _immediately_ above |context|; below any
already-existing bubbles.

That doesn't make sense for a menu-like Widget, so put it back on top.

This only really affects ChromeOS. Windows also uses this touch menu,
but the call to Widget::Show() already causes the Widget to stack on
top again.

Bug: 848327
Change-Id: Icf115c69b0d0ca0e4a5af4ac6ce0f2b0c54c1678
Reviewed-on: https://chromium-review.googlesource.com/1100668Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567938}

ui/views/touchui/touch_selection_menu_runner_views.cc

@@ -131,6 +131,11 @@ TouchSelectionMenuRunnerViews::Menu::Menu(TouchSelectionMenuRunnerViews* owner,
     bounds.AdjustToFit(work_area);
     widget->SetBounds(bounds);
   }
+  // Using BubbleDialogDelegateView engages its CreateBubbleWidget() which
+  // invokes widget->StackAbove(context). That causes the bubble to stack
+  // _immediately_ above |context|; below any already-existing bubbles. That
+  // doesn't make sense for a menu, so put it back on top.
+  widget->StackAtTop();
   widget->Show();
 }