mash: Make system tray bubble close when another window is activated

mash/wm/public/interfaces/container.mojom

Index: mash/wm/public/interfaces/container.mojom
diff --git a/mash/wm/public/interfaces/container.mojom b/mash/wm/public/interfaces/container.mojom
index a9e1685e4c04d1e91d0d8b252e5cd1c6bccaa62d..aacd2e7ee769a14942291cdfb901c8ec3102f558 100644
--- a/mash/wm/public/interfaces/container.mojom
+++ b/mash/wm/public/interfaces/container.mojom
@@ -4,6 +4,9 @@
 
 module mash.wm.mojom;
 
+// Containers for windows. The containers are built in a tree; they are not just
+// stacked in this order. See RootWindowController::CreateContainers() for the
+// hierarchy. 
 enum Container {
   ROOT = 0,
   ALL_USER_BACKGROUND,
@@ -17,6 +20,9 @@ enum Container {
   LOGIN_WINDOWS,
   LOGIN_APP,    // TODO(beng): what about dialog boxes login opens?
   LOGIN_SHELF,
+  // Bubble windows, for example the sysui system tray bubble and the screen
+  // brightness bubble.
+  BUBBLES,
   SYSTEM_MODAL_WINDOWS,
   KEYBOARD,
   MENUS,