chromeos: Convert system tray session length limit to mojo

ash/public/interfaces/session_controller.mojom

Index: ash/public/interfaces/session_controller.mojom
diff --git a/ash/public/interfaces/session_controller.mojom b/ash/public/interfaces/session_controller.mojom
index 4e902cda76cd007c288b7cd314fcb328e7188249..ed42a3ccded5cdb452daa8ff4645ee8e2d1e2c64 100644
--- a/ash/public/interfaces/session_controller.mojom
+++ b/ash/public/interfaces/session_controller.mojom
@@ -6,6 +6,7 @@ module ash.mojom;
 
 import "ash/public/interfaces/user_info.mojom";
 import "components/signin/public/interfaces/account_id.mojom";
+import "mojo/common/time.mojom";
 
 // Matches session_manager::SessionState.
 enum SessionState {
@@ -141,6 +142,15 @@ interface SessionController {
 
   // Notifies that chrome is terminating.
   NotifyChromeTerminating();
+
+  // Creates or updates a notification saying the session length is limited

[msw, 2017/06/06 19:01:04]

optional nit: rephrase as "Adds a countdown timer to the system tray menu and
creates or updates a notification saying the session length is limited (e.g...."

[James Cook, 2017/06/07 00:48:13]

Done.

+  // (e.g. a public session in a library). Adds a countdown timer to the system
+  // tray menu. Setting |length_limit| to zero removes the notification.
+  // NOTE: Chrome enforces the limit, not ash. Ash could enforce it if local
+  // state prefs and user activity monitoring were available under mustash.
+  // http://crbug.com/729808
+  SetSessionLengthLimit(mojo.common.mojom.TimeDelta length_limit,
+                        mojo.common.mojom.TimeTicks start_time);
 };
 
 // Interface for ash to request session service from its client (e.g. Chrome).