chromeos: Make system tray audio item observe CrasAudioHandler directly

ash/common/wm_root_window_controller.cc

Index: ash/common/wm_root_window_controller.cc
diff --git a/ash/common/wm_root_window_controller.cc b/ash/common/wm_root_window_controller.cc
index 4b5048e0eff13099887bf99c7e384f21a9d70bdb..d4f1be97d9218ac924a4ddc0816d970d8b4a278e 100644
--- a/ash/common/wm_root_window_controller.cc
+++ b/ash/common/wm_root_window_controller.cc
@@ -224,6 +224,13 @@ void WmRootWindowController::ShowShelf() {
   shelf->shelf_widget()->status_area_widget()->Show();
 }
 
+SystemTray* WmRootWindowController::GetSystemTray() {
+  ShelfWidget* shelf_widget = GetShelf()->shelf_widget();
+  if (!shelf_widget || !shelf_widget->status_area_widget())
+    return nullptr;
+  return shelf_widget->status_area_widget()->system_tray();
+}
+
 WmWindow* WmRootWindowController::GetContainer(int container_id) {
   return root_->GetChildByShellWindowId(container_id);
 }