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..8130c6ac7050931bd519a7860a2f607c1c6e5047 100644
--- a/ash/system/cast/tray_cast.h
+++ b/ash/system/cast/tray_cast.h
@@ -5,8 +5,10 @@
 #ifndef ASH_SYSTEM_CAST_TRAY_CAST_H_
 #define ASH_SYSTEM_CAST_TRAY_CAST_H_
 
+#include "ash/ash_export.h"
 #include "ash/cast_config_delegate.h"
 #include "ash/shell_observer.h"
+#include "ash/system/cast/tray_cast_testing_interface.h"
 #include "ash/system/tray/system_tray_item.h"
 #include "base/memory/weak_ptr.h"
 
@@ -18,12 +20,22 @@ class CastDetailedView;
 class CastDuplexView;
 }  // namespace tray
 
-class TrayCast : public SystemTrayItem, public ShellObserver {
+class ASH_EXPORT TrayCast : public SystemTrayItem,
+                            public ShellObserver,
+                            public TrayCastTestingInterface {
  public:
   explicit TrayCast(SystemTray* system_tray);
   ~TrayCast() override;
 
  private:
+  // Overridden from TrayCastTestingInterface.
+  bool IsTrayInitializedForTest() const override;
+  bool IsTrayVisibleForTest() const override;
+  bool IsTrayCastViewVisibleForTest() const override;
+  bool IsTraySelectViewVisibleForTest() const override;
+  void StartCastForTest(const std::string& receiver_id) override;
+  void StopCastForTest() override;
+
   // Overridden from SystemTrayItem.
   views::View* CreateTrayView(user::LoginStatus status) override;
   views::View* CreateDefaultView(user::LoginStatus status) override;