mash: Add SystemTrayClient interface, use to show date settings

chrome/browser/ui/ash/system_tray_common.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef CHROME_BROWSER_UI_WEBUI_CAST_CAST_UI_H_
-#define CHROME_BROWSER_UI_WEBUI_CAST_CAST_UI_H_
+#ifndef CHROME_BROWSER_UI_ASH_SYSTEM_TRAY_COMMON_H_
+#define CHROME_BROWSER_UI_ASH_SYSTEM_TRAY_COMMON_H_
 
 #include "base/macros.h"
-#include "base/values.h"
-#include "content/public/browser/web_ui_controller.h"
 
-// The WebUI for chrome://cast
-class CastUI : public content::WebUIController {
+// System tray code shared between classic ash SystemTrayDelegateChromeos and
+// mustash SystemTrayClient.
+class SystemTrayCommon {

[James Cook, 2016/09/22 23:19:18]

This is a class and not a namespace because I anticipate that it will have
state/members very soon. Then SystemTrayDelegateChromeos and SystemTrayClient
will each own an instance.

  public:
-  explicit CastUI(content::WebUI* web_ui);
-  ~CastUI() override;
+  // Shows the settings related to date, timezone etc.
+  static void ShowDateSettings();
+
  private:
-  DISALLOW_COPY_AND_ASSIGN(CastUI);
+  DISALLOW_IMPLICIT_CONSTRUCTORS(SystemTrayCommon);
 };
 
-#endif  // CHROME_BROWSER_UI_WEBUI_CAST_CAST_UI_H_
+#endif  // CHROME_BROWSER_UI_ASH_SYSTEM_TRAY_COMMON_H_