[Sync] Adding storage key concept for ModelTypeServices.

components/sync/api/model_type_service.h

Index: components/sync/api/model_type_service.h
diff --git a/components/sync/api/model_type_service.h b/components/sync/api/model_type_service.h
index 9b2bec714d18b7fcf0dcf8e1d46f093866a11cc6..4b15666f8dd048694327107a3a04e60594d0e663 100644
--- a/components/sync/api/model_type_service.h
+++ b/components/sync/api/model_type_service.h
@@ -34,7 +34,7 @@ class SYNC_EXPORT ModelTypeService {
  public:
   typedef base::Callback<void(syncer::SyncError, std::unique_ptr<DataBatch>)>
       DataCallback;
-  typedef std::vector<std::string> ClientTagList;
+  typedef std::vector<std::string> StorageKeyList;
   typedef base::Callback<std::unique_ptr<ModelTypeChangeProcessor>(
       syncer::ModelType type,
       ModelTypeService* service)>
@@ -75,15 +75,28 @@ class SYNC_EXPORT ModelTypeService {
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityChangeList entity_changes) = 0;
 
-  // Asynchronously retrieve the corresponding sync data for |client_tags|.
-  virtual void GetData(ClientTagList client_tags, DataCallback callback) = 0;
+  // Asynchronously retrieve the corresponding sync data for |storage_keys|.
+  virtual void GetData(StorageKeyList storage_keys, DataCallback callback) = 0;
 
   // Asynchronously retrieve all of the local sync data.
   virtual void GetAllData(DataCallback callback) = 0;
 
-  // Get or generate a client tag for |entity_data|.
+  // Get or generate a client tag for |entity_data|. This must be the same tag
+  // that was/would have been generated in the SyncableService/Directory world.

[maxbogue, 2016/08/10 23:56:02]

"world for backward-compatibility with pre-USS clients."

[skym, 2016/08/11 16:28:47]

Done.

+  // The only time this theoretically needs to be called is on the creation of
+  // local data, however we also call and verify on remote data to make sure the

[maxbogue, 2016/08/10 23:56:02]

"however it is also used to verify the hash of remote data."

[skym, 2016/08/11 16:28:47]

Done.

+  // hash we would generate is the same. If a data type was never launched
+  // pre-USS, then method does not need to be different from GetStorageKey().
   virtual std::string GetClientTag(const EntityData& entity_data) = 0;
 
+  // Get or generate a storage key for |entity_data|. This will only ever be
+  // called once when first encountering a remote entity. Local changes should

[maxbogue, 2016/08/10 23:56:02]

"Local changes will provide their storage keys directly to Put instead of using
this method."

[skym, 2016/08/11 16:28:47]

Done.

+  // have their storage keys proactivly supplied to the processor instead of
+  // using this method. Theoretically this function doesn't need to be stable
+  // across multiple calls on the same or different clients, but to keep things
+  // simple, it probably should be.
+  virtual std::string GetStorageKey(const EntityData& entity_data) = 0;
+
   // Overridable notification for when the processor is set. This is typically
   // when the service should start loading metadata and then subsequently giving
   // it to the processor.