Add UI for cast system tray integration

ash/system/chromeos/screen_security/screen_capture_tray_item.h

Index: ash/system/chromeos/screen_security/screen_capture_tray_item.h
diff --git a/ash/system/chromeos/screen_security/screen_capture_tray_item.h b/ash/system/chromeos/screen_security/screen_capture_tray_item.h
index 5c6ed8aeac9141b1e992bb9e4064453d7b1374f4..5b8dd7de0e09a745b3245d7e68bccb028d440492 100644
--- a/ash/system/chromeos/screen_security/screen_capture_tray_item.h
+++ b/ash/system/chromeos/screen_security/screen_capture_tray_item.h
@@ -5,6 +5,7 @@
 #ifndef ASH_SYSTEM_CHROMEOS_SCREEN_CAPTURE_SCREEN_CAPTURE_TRAY_ITEM_H_
 #define ASH_SYSTEM_CHROMEOS_SCREEN_CAPTURE_SCREEN_CAPTURE_TRAY_ITEM_H_
 
+#include "ash/shell_observer.h"
 #include "ash/system/chromeos/screen_security/screen_capture_observer.h"
 #include "ash/system/chromeos/screen_security/screen_tray_item.h"
 
@@ -15,7 +16,8 @@ class View;
 namespace ash {
 
 class ASH_EXPORT ScreenCaptureTrayItem : public ScreenTrayItem,
-                                         public ScreenCaptureObserver {
+                                         public ScreenCaptureObserver,
+                                         public ShellObserver {
  public:
   explicit ScreenCaptureTrayItem(SystemTray* system_tray);
   ~ScreenCaptureTrayItem() override;
@@ -35,7 +37,11 @@ class ASH_EXPORT ScreenCaptureTrayItem : public ScreenTrayItem,
       const base::string16& screen_capture_status) override;
   void OnScreenCaptureStop() override;
 
+  // Overridden from ShellObserver.
+  void OnCastingSessionStartedOrStopped(bool started) override;
+
   base::string16 screen_capture_status_;
+  bool is_casting_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenCaptureTrayItem);
 };