Keep the mouse cursor within desktop

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..926aac119c4f9e2e43086d13e2643e1345b30bc2 100644
--- a/ash/monitor/multi_monitor_manager.cc
+++ b/ash/monitor/multi_monitor_manager.cc
@@ -45,8 +45,8 @@ MultiMonitorManager::MultiMonitorManager() {
 }
 
 MultiMonitorManager::~MultiMonitorManager() {
-  // All monitors must have been deleted when monitor manager is deleted.
-  CHECK(!monitors_.size());
+  // All monitors must have been deleted when root windows are deleted.
+  DCHECK(!monitors_.size());
 }
 
 // static