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..ed0fdf55684bed5e430e3b7d191b9e6690e2c49d 100644
--- a/ash/public/interfaces/system_tray.mojom
+++ b/ash/public/interfaces/system_tray.mojom
@@ -6,9 +6,17 @@ module ash.mojom;
 
 // Allows clients (e.g. Chrome browser) to control the ash system tray menu.
 interface SystemTray {
+  // Sets the client interface.
+  SetClient(SystemTrayClient system_tray_client);
+
   // Sets the clock to use 24 hour time formatting if |use_24_hour| is true.
   // Otherwise sets 12 hour time formatting.
   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();
+};