Add shutdown notification and service refs to SyncServiceObserver.

components/sync/driver/sync_service_observer.h

Index: components/sync/driver/sync_service_observer.h
diff --git a/components/sync/driver/sync_service_observer.h b/components/sync/driver/sync_service_observer.h
index 0c6e68e361054231d818009a91ea21f3a0b89c91..b6f264b981f63ad931e2fcd50b0c7878f906da69 100644
--- a/components/sync/driver/sync_service_observer.h
+++ b/components/sync/driver/sync_service_observer.h
@@ -7,6 +7,8 @@
 
 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 {
@@ -28,11 +30,15 @@ class SyncServiceObserver {
 
   // 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() {}
 };