[Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/api/syncable_service.h

Index: sync/api/syncable_service.h
diff --git a/sync/api/syncable_service.h b/sync/api/syncable_service.h
index f83a017c2da14f1fa08f96b4bcdcc9b760d1c876..03b377f196434eb51329677544389c91fa7a0a88 100644
--- a/sync/api/syncable_service.h
+++ b/sync/api/syncable_service.h
@@ -72,9 +72,9 @@ class SYNC_EXPORT SyncableService
   // that time GetAttachmentStore returns NULL then datatype is considered not
   // using attachments and all attempts to upload/download attachments will
   // fail. Default implementation returns NULL. Datatype that uses sync
-  // attachemnts should create attachment store and implement GetAttachmentStore
+  // attachments should create attachment store and implement GetAttachmentStore
   // to return pointer to it.
-  virtual scoped_refptr<AttachmentStore> GetAttachmentStore();
+  virtual AttachmentStore* GetAttachmentStore();

[maniscalco, 2015/03/06 23:15:03]

As we talked about in person, it would be great if SyncableService could create
the AttachmentStoreForSync and return it by scoped_ptr.  That way, we'd never
have to worry about the lifetime of the object returned by GetAttachmentStore()
[Should I free it?  Can I hold on to the pointer and use it later?  How much
later?]

[pavely, 2015/03/09 18:09:29]

Done.

 
   // Called by sync to provide AttachmentService to be used to download
   // attachments.