Add update available icon in system tray

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 f1108c49d4f0b7e209de134d65607456a0303627..1895a694a7942d3c76590df877c127593d48907c 100644
--- a/ash/public/interfaces/system_tray.mojom
+++ b/ash/public/interfaces/system_tray.mojom
@@ -34,6 +34,11 @@ interface SystemTray {
   ShowUpdateIcon(UpdateSeverity severity,
                  bool factory_reset_required,
                  UpdateType update_type);
+
+  // Shows an icon in the system tray indicating that a software update is
+  // available but user's agreement is required as current connection is
+  // cellular. Once shown the icon persists until reboot.
+  ShowUpdateOverCellularAvailableIcon();
 };
 
 // Allows ash system tray to control a client (e.g. Chrome browser). Requests
@@ -57,6 +62,9 @@ interface SystemTrayClient {
   // Shows the settings related to date, timezone etc.
   ShowDateSettings();
 
+  // Show the about chrome OS page.
+  ShowAboutChromeOS();

[stevenjb, 2017/05/15 18:06:45]

nit: Put this next to 'ShowHelp', and update the comments as follows. 

// Show the 'About Chrome OS' page in the Settings UI.
ShowAboutChromeOS();

// Show the Chromebook help app.
ShowHelp();

(The difference between 'help' and 'about' is often confused, e.g. even though
the Settings page is 'About Chrome OS', the URL for it is (for historical
reasons) chrome://settings/help. That's why I want to clarify these here.)

[weidongg, 2017/05/16 19:46:08]

Done.

+
   // Shows the dialog to set system time, date, and timezone.
   ShowSetTimeDialog();
 
@@ -121,4 +129,7 @@ interface SystemTrayClient {
 
   // Attempts to restart the system for update.
   RequestRestartForUpdate();
+
+  // Attempts to check for update;
+  RequestUpdateCheck();
 };