ash: Do not allow minimizing windows in login screen.

ash/wm/toplevel_window_event_handler.cc

Index: ash/wm/toplevel_window_event_handler.cc
diff --git a/ash/wm/toplevel_window_event_handler.cc b/ash/wm/toplevel_window_event_handler.cc
index 4320a7ff52350477cf22fd5c3208a0566348ce1b..33360d157fa66937ca261a63ec64f737b8f1b63d 100644
--- a/ash/wm/toplevel_window_event_handler.cc
+++ b/ash/wm/toplevel_window_event_handler.cc
@@ -212,7 +212,8 @@ void ToplevelWindowEventHandler::OnGestureEvent(ui::GestureEvent* event) {
       if (fabs(event->details().velocity_y()) >
           kMinVertVelocityForWindowMinimize) {
         // Minimize/maximize.
-        if (event->details().velocity_y() > 0) {
+        if (event->details().velocity_y() > 0 &&
+            wm::CanMinimizeWindow(target)) {
           wm::MinimizeWindow(target);
           SetWindowAlwaysRestoresToRestoreBounds(target, true);
           SetRestoreBoundsInParent(target, pre_drag_window_bounds_);