Notify newly added observer of existing devices

Newly added observers are not currently notified of existing
devices. If ScanDevices() is called before all observers are
registered then some observers will miss device events.

Bug: 70595920
Test: Observers receive OnDeviceEvent() correctly when registered
Change-Id: Ia04b894773ac676409e2d6d12b31e3b3801eb140
Reviewed-on: https://chromium-review.googlesource.com/841424Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Anthony Zhang <antz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526039}

ui/events/ozone/device/device_manager_manual.cc

@@ -46,6 +46,11 @@ void DeviceManagerManual::ScanDevices(DeviceEventObserver* observer) {
 
 void DeviceManagerManual::AddObserver(DeviceEventObserver* observer) {
   observers_.AddObserver(observer);
+  // Notify the new observer about existing devices.
+  for (const auto& path : devices_) {
+    DeviceEvent event(DeviceEvent::INPUT, DeviceEvent::ADD, path);
+    observer->OnDeviceEvent(event);
+  }
 }
 
 void DeviceManagerManual::RemoveObserver(DeviceEventObserver* observer) {