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

ash/common/system/update/update_observer.h

 // Copyright (c) 2012 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.
 
 #ifndef ASH_COMMON_SYSTEM_UPDATE_UPDATE_OBSERVER_H_
 #define ASH_COMMON_SYSTEM_UPDATE_UPDATE_OBSERVER_H_
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
-struct UpdateInfo;
+namespace mojom {
+enum class UpdateSeverity;
+}
 
 class ASH_EXPORT UpdateObserver {
  public:
+  virtual void ShowUpdateIcon(mojom::UpdateSeverity severity,

[msw, 2016/12/09 23:41:51]

It's somewhat odd that we have an observer pattern for the
SystemTray[Controller|Notifier] to reach TrayUpdate's implementation, especially
since that's the only observer, and the observer function is more of an
imperative ("Show it") than a heads up ("An Update is recommended"). Is it
possible and desirable to simplify this (perhaps in a followup)? It seems like
SystemTrayController could use something like SystemTray::GetTrayUpdate() to
replace the observation and avoid the notifier altogether.

[James Cook, 2016/12/12 18:15:12]

I removed the observer. This revealed a problem -- if an external monitor was
connected mid-session then its TrayUpdate would have stale info about the update
state. I fixed that by making the cached update info in TrayUpdate static.

+                              bool factory_reset_required) = 0;
+
+ protected:
   virtual ~UpdateObserver() {}
-
-  virtual void OnUpdateRecommended(const UpdateInfo& info) = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_UPDATE_UPDATE_OBSERVER_H_