Apply new MD shadows to CrOS tray bubbles.

ui/views/bubble/bubble_border.h

Index: ui/views/bubble/bubble_border.h
diff --git a/ui/views/bubble/bubble_border.h b/ui/views/bubble/bubble_border.h
index ec1e25a88b5895e28593612c87c76494130f8dbc..06c2bb4ae11917d327334b4589723a4c96464888 100644
--- a/ui/views/bubble/bubble_border.h
+++ b/ui/views/bubble/bubble_border.h
@@ -100,6 +100,8 @@ class VIEWS_EXPORT BubbleBorder : public Border {
     NO_SHADOW_OPAQUE_BORDER,
     BIG_SHADOW,
     SMALL_SHADOW,
+    // NO_ASSETS borders don't draw a stroke or a shadow. This is used for
+    // platforms that provide their own shadows.
     NO_ASSETS,
     SHADOW_COUNT,
   };