[Video Capture, Chrome OS] Notify Chrome when devices has changed

BUG=b:118418068,b:118408452,b:64996728,b:77833131
TEST=Check the external camera is shown in settings.

Change-Id: I83604f5114d4dcd317c281cf63e9fd50ef619d9f
Reviewed-on: https://chromium-review.googlesource.com/c/1326342Reviewed-by: Ricky Liang <jcliang@chromium.org>
Commit-Queue: Shik Chen <shik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606453}

media/capture/video/chromeos/camera_hal_delegate.cc

@@ -16,6 +16,7 @@
 #include "base/process/launch.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/string_split.h"
+#include "base/system/system_monitor.h"
 #include "media/capture/video/chromeos/camera_buffer_factory.h"
 #include "media/capture/video/chromeos/camera_hal_dispatcher_impl.h"
 #include "media/capture/video/chromeos/camera_metadata_utils.h"
@@ -454,6 +455,12 @@ void CameraHalDelegate::CameraDeviceStatusChange(
     default:
       NOTREACHED() << "Unexpected new status " << new_status;
   }
+  base::SystemMonitor* monitor = base::SystemMonitor::Get();
+  // |monitor| might be nullptr in unittest.
+  if (monitor) {
+    monitor->ProcessDevicesChanged(
+        base::SystemMonitor::DeviceType::DEVTYPE_VIDEO_CAPTURE);
+  }
 }
 
 void CameraHalDelegate::TorchModeStatusChange(