[GStreamer][1.28] Ensure synchronous initialization of the device monitor

https://bugs.webkit.org/show_bug.cgi?id=308961

Reviewed by Xabier Rodriguez-Calvar.

In GStreamer 1.28 device providers start asynchronously and the monitor posts a message on the bus
once it has finished starting-up. We need the initial list of devices right away after starting, so
wait for the message to be posted on the bus before attempting to get the list of devices.

* Source/WebCore/platform/mediastream/gstreamer/GStreamerCaptureDeviceManager.cpp:
(WebCore::GStreamerCaptureDeviceManager::refreshCaptureDevices):

Canonical link: https://commits.webkit.org/308531@main

Author: philn

Source/WebCore/platform/mediastream/gstreamer/GStreamerCaptureDeviceManager.cpp

@@ -339,6 +339,14 @@ void GStreamerCaptureDeviceManager::refreshCaptureDevices()
             return;
         }
 
+#if GST_CHECK_VERSION(1, 28, 0)
+        {
+            auto bus = adoptGRef(gst_device_monitor_get_bus(m_deviceMonitor.get()));
+            [[maybe_unused]] auto message = adoptGRef(gst_bus_poll(bus.get(), GST_MESSAGE_DEVICE_MONITOR_STARTED, GST_CLOCK_TIME_NONE));
+            GST_DEBUG_OBJECT(m_deviceMonitor.get(), "Device provider(s) successfully started");
+        }
+#endif
+
         monitorBus = true;
     }