ash: Use system tray mojo interface to show system update tray icon

ash/public/interfaces/update.mojom

Index: ash/public/interfaces/update.mojom
diff --git a/ash/public/interfaces/update.mojom b/ash/public/interfaces/update.mojom
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index 0000000000000000000000000000000000000000..dd52e5033cadad36ae881dd3f6c896f4336caf5e
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+++ b/ash/public/interfaces/update.mojom
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module ash.mojom;
+
+// Urgency of a pending software update. Sets the system tray update icon color.
+// TODO(jamescook): UpgradeDetector::UpgradeNotificationAnnoyanceLevel could be
+// replaced with this if this moves into a component shared with non-ash chrome.
+enum UpdateSeverity {
+  NONE,
+  LOW,
+  ELEVATED,
+  HIGH,
+  SEVERE,
+  CRITICAL,
+};