Don't delete Primary root window and monitor

ash/monitor/multi_monitor_manager.cc

Index: ash/monitor/multi_monitor_manager.cc
diff --git a/ash/monitor/multi_monitor_manager.cc b/ash/monitor/multi_monitor_manager.cc
index 8ed2de9be05718c1ce4593dacf59e7ff2db0cbf6..36242af35ee20106125abfdd925bb9e90de7acb2 100644
--- a/ash/monitor/multi_monitor_manager.cc
+++ b/ash/monitor/multi_monitor_manager.cc
@@ -90,8 +90,10 @@ void MultiMonitorManager::OnNativeMonitorsChanged(
       NotifyMonitorAdded(monitor);
     }
   } else {
-    // Monitors are removed.
-    while (monitors_.size() > new_monitors.size()) {
+    // Monitors are removed. We keep the monitor for the primary
+    // monitor (at index 0) because it needs the monitor information
+    // even if it doesn't exit.
+    while (monitors_.size() > new_monitors.size() && monitors_.size() > 1) {
       Monitor* monitor = monitors_.back();
       // Monitor object is deleted in OnWindowDestroying.
       NotifyMonitorRemoved(monitor);