mash: Add SystemTrayClient interface, use to show date settings

ash/public/interfaces/system_tray.mojom

Index: ash/public/interfaces/system_tray.mojom
diff --git a/ash/public/interfaces/system_tray.mojom b/ash/public/interfaces/system_tray.mojom
index 14c4f949e7b33ef3c662cc75469c68c2c8e4d071..69c0b83693e00b44085f05a89070760e75c15082 100644
--- a/ash/public/interfaces/system_tray.mojom
+++ b/ash/public/interfaces/system_tray.mojom
@@ -11,4 +11,9 @@ interface SystemTray {
   SetUse24HourClock(bool use_24_hour);
 };
 
-// TODO(jamescook): SystemTrayDelegate to communicate from ash to chrome.
+// Allows ash system tray to control a client (e.g. Chrome browser). Requests
+// often involve preferences or web UI that is not available to ash.
+interface SystemTrayClient {
+  // Shows the settings related to date, timezone etc.
+  ShowDateSettings();
+};