Add InvalidationStateTracker::Forget() to erase an entry from storage.

sync/notifier/registration_manager.h

Index: sync/notifier/registration_manager.h
diff --git a/sync/notifier/registration_manager.h b/sync/notifier/registration_manager.h
index 9d0036590441064db775142e1f5868928a2d1b61..01ae7b9c807327a54a88d7a51866d453037a9fbc 100644
--- a/sync/notifier/registration_manager.h
+++ b/sync/notifier/registration_manager.h
@@ -68,8 +68,9 @@ class RegistrationManager : public base::NonThreadSafe {
   virtual ~RegistrationManager();
 
   // Registers all object IDs included in the given set (that are not
-  // already disabled) and unregisters all other object IDs.
-  void UpdateRegisteredIds(const ObjectIdSet& ids);
+  // already disabled) and unregisters all other object IDs. The return value is
+  // the set of IDs that was unregistered.
+  ObjectIdSet UpdateRegisteredIds(const ObjectIdSet& ids);

[akalin, 2012/08/22 19:57:30]

what do you think of making GetRegisteredIdsForTest() not be just for test, and
moving the calculation of the types to forget to the caller?  I feel like the
return value of this function isn't very intuitive.

Also, I have another CL that needs GetRegisteredIds(), anyway.

[dcheng, 2012/08/24 01:35:25]

I'm not opposed to exposing it publicly.

However, making the caller makes the flow kind of funky:
Caller gets the registered ids and saves it somewhere.
Callee gets the registered ids, then diffs it against the set passed in, updates
registrations, etc.
Caller then does the exact same diff and gets the exact same result set.

I do agree with comment that it's not intuitive though. I'll think about it more
and see if I can come up with something that is better.

[akalin, 2012/08/29 17:33:54]

Yeah, you're right about the funky flow.  Actually, since this is just the
RegistrationManager, which is pretty much an internal class, I think keeping the
unregistered set as a return value is the least bad option.

 
   // Marks the registration for the |id| lost and re-registers
   // it (unless it's disabled).