Multiprofile security: Show a warning messagebox when desktop casting/sharing is turned on upon user

ash/system/user/tray_user_separator.cc

Index: ash/system/user/tray_user_separator.cc
diff --git a/ash/system/user/tray_user_separator.cc b/ash/system/user/tray_user_separator.cc
index 7058d2fbb9507645b1a762a95b9c78cc4256785d..d152a3b33aecf79ba3e51427c074ee434de644d4 100644
--- a/ash/system/user/tray_user_separator.cc
+++ b/ash/system/user/tray_user_separator.cc
@@ -26,8 +26,10 @@ views::View* TrayUserSeparator::CreateDefaultView(user::LoginStatus status) {
   const SessionStateDelegate* session_state_delegate =
       Shell::GetInstance()->session_state_delegate();
 
-  // If the screen is locked, or only a single user is shown, show nothing.
+  // If the screen is locked, a system modal dialog or a single user is shown,
+  // show nothing.
   if (session_state_delegate->IsUserSessionBlocked() ||
+      Shell::GetInstance()->IsSystemModalWindowOpen() ||
       session_state_delegate->NumberOfLoggedInUsers() < 2)
     return NULL;