mash: Add SystemTrayClient interface, use to show date settings

ash/mus/system_tray_delegate_mus.h

Index: ash/mus/system_tray_delegate_mus.h
diff --git a/ash/mus/system_tray_delegate_mus.h b/ash/mus/system_tray_delegate_mus.h
index e0a6270a287fc5deac4a19a590e6823e98f07a7c..12413532d4f8c2cc857a708be254d59bb0037a00 100644
--- a/ash/mus/system_tray_delegate_mus.h
+++ b/ash/mus/system_tray_delegate_mus.h
@@ -28,10 +28,15 @@ class SystemTrayDelegateMus : public DefaultSystemTrayDelegate,
  private:
   // SystemTrayDelegate:
   base::HourClockType GetHourClockType() const override;
+  void ShowDateSettings() override;
 
   // mojom::SystemTray:
+  void SetClient(mojom::SystemTrayClientPtr system_tray_client) override;
   void SetUse24HourClock(bool use_24_hour) override;
 
+  // The client is usually chrome browser.
+  mojom::SystemTrayClientPtr system_tray_client_;
+
   // 12 or 24 hour display.
   base::HourClockType hour_clock_type_;