Update comments around InvalidationHandler::GetOwnerName()

GetOwnerName() must return unique value for each handler within
the same invalidation service. This is currently not the case for
CloudPolicyInvalidator.

This change documents this contract to avoid further API misuse and
updates TODO for codepath depending on this contract.

Bug: 1020117, 1049591
Change-Id: I7fbda6569bf6837c274be753fee5a472bbe2fff4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2049973Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Cr-Commit-Position: refs/heads/master@{#740273}

components/invalidation/impl/invalidator_registrar_with_memory.cc

@@ -139,8 +139,10 @@ bool InvalidatorRegistrarWithMemory::UpdateRegisteredTopics(
 
   DictionaryPrefUpdate update(prefs_, kTopicsToHandler);
   base::Value* pref_data = update->FindDictKey(sender_id_);
-  // TODO(crbug.com/1020117): This does currently *not* remove subscribed topics
-  // which are not registered, but it almost certainly should.
+  // TODO(crbug.com/1020117): This does currently *not* remove subscribed
+  // topics which are not registered, but it almost certainly should. It
+  // requires GetOwnerName() to return unique value for each handler, which is
+  // currently not the case for CloudPolicyInvalidator (see crbug.com/1049591).
   auto to_unregister = FindRemovedTopics(old_topics, topics);
   for (const auto& topic : to_unregister) {
     pref_data->RemoveKey(topic);

components/invalidation/public/invalidation_handler.h

@@ -28,6 +28,10 @@ class INVALIDATION_EXPORT InvalidationHandler {
   virtual void OnIncomingInvalidation(
       const ObjectIdInvalidationMap& invalidation_map) = 0;
 
+  // Returned value must be unique for the handlers using the same invalidation
+  // service.
+  // TODO(crbug.com/1049591): this is currently not the case for
+  // CloudPolicyInvalidator.
   virtual std::string GetOwnerName() const = 0;
 
   // Called on change of |client_id|. Client id is used to identify the