Add UI for cast system tray integration

ash/system/tray/system_tray_item.h

Index: ash/system/tray/system_tray_item.h
diff --git a/ash/system/tray/system_tray_item.h b/ash/system/tray/system_tray_item.h
index 61d214eb558c3db39c4ff4c3522bb09dd47813d7..d2c1f95cb6f67ee573a4523bf42ad34f87ad6aba 100644
--- a/ash/system/tray/system_tray_item.h
+++ b/ash/system/tray/system_tray_item.h
@@ -35,9 +35,9 @@ class ASH_EXPORT SystemTrayItem {
   // avoid this behavior, then it should not be a TrayItemView.
   virtual views::View* CreateTrayView(user::LoginStatus status);
 
-  // Returns a view for the item to be displayed in the list. This view can be
-  // displayed with a number of other tray items, so this should not be too
-  // big.
+  // Returns a view for the item to be displayed in the popup list after
+  // clicking the system tray. This view can be displayed with a number of other
+  // tray items, so this should not be too big.
   virtual views::View* CreateDefaultView(user::LoginStatus status);
 
   // Returns a detailed view for the item. This view is displayed standalone.