Add support code to test the cast system tray item.

ash/system/cast/tray_cast.h

Index: ash/system/cast/tray_cast.h
diff --git a/ash/system/cast/tray_cast.h b/ash/system/cast/tray_cast.h
index 04a4bc28a802aebee55c40818e4a36ed0e16a31e..31c456b188435ea83f73af0ac364f29215b99e51 100644
--- a/ash/system/cast/tray_cast.h
+++ b/ash/system/cast/tray_cast.h
@@ -18,12 +18,18 @@ class CastDetailedView;
 class CastDuplexView;
 }  // namespace tray
 
-class TrayCast : public SystemTrayItem, public ShellObserver {
+class ASH_EXPORT TrayCast : public SystemTrayItem, public ShellObserver {
  public:
   explicit TrayCast(SystemTray* system_tray);
   ~TrayCast() override;
 
  private:
+  // Helper/utility methods for testing.
+  friend class TrayCastTestAPI;
+  void StartCastForTest(const std::string& receiver_id);
+  void StopCastForTest();
+  enum ChildViewIds { TRAY_VIEW, SELECT_VIEW, CAST_VIEW };
+
   // Overridden from SystemTrayItem.
   views::View* CreateTrayView(user::LoginStatus status) override;
   views::View* CreateDefaultView(user::LoginStatus status) override;