ozone: Initialize a subsystem only if necessary.

ui/events/ozone/device/udev/device_manager_udev.cc

Index: ui/events/ozone/device/udev/device_manager_udev.cc
diff --git a/ui/events/ozone/device/udev/device_manager_udev.cc b/ui/events/ozone/device/udev/device_manager_udev.cc
index 74e50997265b6f5aef539db03f3729761e6a7f44..47244a46964f5dafa645f0ebf3a3bb2b27809922 100644
--- a/ui/events/ozone/device/udev/device_manager_udev.cc
+++ b/ui/events/ozone/device/udev/device_manager_udev.cc
@@ -78,10 +78,16 @@ scoped_udev_monitor UdevCreateMonitor(struct udev* udev) {
 
 }  // namespace
 
-DeviceManagerUdev::DeviceManagerUdev()
-  : udev_(UdevCreate()),
-    monitor_(UdevCreateMonitor(udev_.get())) {
+DeviceManagerUdev::DeviceManagerUdev() : udev_(UdevCreate()) {
+}
+
+DeviceManagerUdev::~DeviceManagerUdev() {
+}
 
+void DeviceManagerUdev::CreateMonitor() {
+  if (monitor_)
+    return;
+  monitor_ = UdevCreateMonitor(udev_.get());
   if (monitor_) {
     int fd = udev_monitor_get_fd(monitor_.get());
     CHECK_GT(fd, 0);
@@ -90,9 +96,9 @@ DeviceManagerUdev::DeviceManagerUdev()
   }
 }
 
-DeviceManagerUdev::~DeviceManagerUdev() {}
-
 void DeviceManagerUdev::ScanDevices(DeviceEventObserver* observer) {
+  CreateMonitor();
+
   scoped_udev_enumerate enumerate(udev_enumerate_new(udev_.get()));
   if (!enumerate)
     return;