chromeos: Make system tray audio item observe CrasAudioHandler directly

ash/common/wm_root_window_controller.h

Index: ash/common/wm_root_window_controller.h
diff --git a/ash/common/wm_root_window_controller.h b/ash/common/wm_root_window_controller.h
index 43eca2e44dc27590d21ca4ec0ab5a650978e9172..b0f75c78a3ecfe7e8f99b735703d4d0cacb22f8a 100644
--- a/ash/common/wm_root_window_controller.h
+++ b/ash/common/wm_root_window_controller.h
@@ -31,6 +31,7 @@ class AnimatingWallpaperWidgetController;
 class DockedWindowLayoutManager;
 class PanelLayoutManager;
 class SystemModalContainerLayoutManager;
+class SystemTray;
 class WallpaperWidgetController;
 class WmShelf;
 class WmShell;
@@ -103,6 +104,9 @@ class ASH_EXPORT WmRootWindowController {
   // TODO(jamescook): Eliminate this and handle show via Shelf.
   void ShowShelf();
 
+  // Returns the system tray controller. May be null for external displays.
+  SystemTray* GetSystemTray();
+
   // Returns the window associated with this WmRootWindowController.
   virtual WmWindow* GetWindow() = 0;