Add shutdown notification and service refs to SyncServiceObserver.

components/sync/driver/sync_service_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 COMPONENTS_SYNC_DRIVER_SYNC_SERVICE_OBSERVER_H_
 #define COMPONENTS_SYNC_DRIVER_SYNC_SERVICE_OBSERVER_H_
 
 namespace syncer {
 
+class SyncService;
+
 // Various UI components such as the New Tab page can be driven by observing
 // the SyncService through this interface.
 class SyncServiceObserver {
  public:
   // When one of the following events occurs, OnStateChanged() is called.
   // Observers should query the service to determine what happened.
   // - We initialized successfully.
   // - The sync servers are unavailable at this time.
   // - Credentials are now in flight for authentication.
   // - The data type configuration has started or ended.
   // - Sync shut down.
   // - Sync errors (passphrase, auth, unrecoverable, actionable, etc.).
   // - Encryption changes.
   virtual void OnStateChanged() = 0;
 
   // If a client wishes to handle sync cycle completed events in a special way,
   // they can use this function.  By default, it re-routes to OnStateChanged().
   virtual void OnSyncCycleCompleted();
 
   // Called when the sync service has finished the datatype configuration
   // process.
-  virtual void OnSyncConfigurationCompleted() {}
+  virtual void OnSyncConfigurationCompleted(SyncService* sync) {}

[skym, 2017/01/26 17:00:37]

So I'm not sure what other checks/events you're listening to, but I don't think
is going to suffice for you. I ran into a similar problem recently when I wanted
to remove sessions suggestions when the user signs out/disables sync.
https://codereview.chromium.org/2578293002/diff/160001/components/ntp_snippet...

Configuration happens when you change the set of types enabled types. But you
need to still have at least one enabled type for configuration to run. If you
end up disabling all the types or signing out, this event doesn't get fired.

The solution is to listen to OnStateChange(). I think we want to update all
these functions to pass a SyncService* for consistency anyways. Though I'd be
willing to do this if you want, you're already improving things significantly
imo.

[Steven Holte, 2017/01/26 20:31:47]

Added SyncService* to all functions.
Also made a default impl here for OnStateChanged, so it can be omitted in some
subclasses.

 
   // Called when a foreign session has been updated.
   virtual void OnForeignSessionUpdated() {}
 
+  // Called when the sync service is being shutdown permanently, so that
+  // longer-lived observers can drop references to it.
+  virtual void OnSyncShutdown(SyncService* sync) {}
+
  protected:
   virtual ~SyncServiceObserver() {}
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_SYNC_SERVICE_OBSERVER_H_