Use mojo SystemTray interfaces for both mash and classic ash

chrome/browser/ui/ash/system_tray_common.h

Index: chrome/browser/ui/ash/system_tray_common.h
diff --git a/chrome/browser/ui/ash/system_tray_common.h b/chrome/browser/ui/ash/system_tray_common.h
index 29ddd5205f4628fa150af212d072bf9ae815b174..05d92ad948cd691d13999073f77a962291d91082 100644
--- a/chrome/browser/ui/ash/system_tray_common.h
+++ b/chrome/browser/ui/ash/system_tray_common.h
@@ -11,6 +11,7 @@
 
 // System tray code shared between classic ash SystemTrayDelegateChromeos and
 // mustash SystemTrayClient.
+// TODO: all this code moves directly in SystemTrayClient. Less indirection.
 class SystemTrayCommon {
  public:
   static const char kDisplaySettingsSubPageName[];
@@ -19,9 +20,6 @@ class SystemTrayCommon {
   // Shows general settings UI.
   static void ShowSettings();
 
-  // Shows the settings related to date, timezone etc.
-  static void ShowDateSettings();
-
   // Shows settings related to multiple displays.
   static void ShowDisplaySettings();