Remove LAYER_NOT_DRAWN parameter for opaque tray bubbles

The parameter causes UI elements without their own layers to
be not visible on these bubbles.

Change-Id: Ie6a13cb30151fe827623bc888d67f517cb2f0477
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017910Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734721}

ash/system/tray/tray_bubble_view.cc

@@ -257,7 +257,7 @@ TrayBubbleView::TrayBubbleView(const InitParams& init_params)
     // Create a layer so that the layer for FocusRing stays in this view's
     // layer. Without it, the layer for FocusRing goes above the
     // NativeViewHost and may steal events.
-    SetPaintToLayer(ui::LAYER_NOT_DRAWN);
+    SetPaintToLayer();
   }
 
   auto layout = std::make_unique<BottomAlignedBoxLayout>(this);