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

ash/common/system/update/update_observer.h

Index: ash/common/system/update/update_observer.h
diff --git a/ash/common/system/update/update_observer.h b/ash/common/system/update/update_observer.h
index 3912f0dd0429cd9715471ee49e38fe4a6d1b03f8..ceea5a42c99c4e48e967f212927227267db605bb 100644
--- a/ash/common/system/update/update_observer.h
+++ b/ash/common/system/update/update_observer.h
@@ -9,13 +9,17 @@
 
 namespace ash {
 
-struct UpdateInfo;
+namespace mojom {
+enum class UpdateSeverity;
+}
 
 class ASH_EXPORT UpdateObserver {
  public:
-  virtual ~UpdateObserver() {}
+  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;
 
-  virtual void OnUpdateRecommended(const UpdateInfo& info) = 0;
+ protected:
+  virtual ~UpdateObserver() {}
 };
 
 }  // namespace ash