[Sync] Introduce AttachmentStoreForSync class

In this change:
- Add AttachmentStoreForSync class. It has functions that will be called by AttachmentServiceImpl to manage sync’s references to attachments.
- Change AttachmentServiceImpl to call those functions. The rule is that for all attachments that we start uploading call AddSyncReference, for all attachments that we no longer upload (for whatever reason) call DropSyncReference.

Couple of notes:
- There is no storage implementation for those functions, it will come in the next change.
- There is growing confusion between AttachmentIdSet and AttachmentIdList. I’ll resolve this next week in a separate change.

BUG=457735
R=maniscalco@chromium.org
TEST=No user facing behavior. Change covered by sync_unit_tests.

Review URL: https://codereview.chromium.org/1002263005

Cr-Commit-Position: refs/heads/master@{#322263}

1 files changed, 3 insertions, 2 deletions

diff --git a/sync/api/fake_syncable_service.cc b/sync/api/fake_syncable_service.cc
index d2177e9..342dd2d 100644
--- a/sync/api/fake_syncable_service.cc
+++ b/sync/api/fake_syncable_service.cc
@@ -70,9 +70,10 @@ SyncError FakeSyncableService::ProcessSyncChanges(
   return process_sync_changes_error_;
 }
 
-scoped_ptr<AttachmentStore> FakeSyncableService::GetAttachmentStoreForSync() {
+scoped_ptr<AttachmentStoreForSync>
+FakeSyncableService::GetAttachmentStoreForSync() {
   return attachment_store_ ? attachment_store_->CreateAttachmentStoreForSync()
-                           : scoped_ptr<AttachmentStore>();
+                           : scoped_ptr<AttachmentStoreForSync>();
 }
 
 void FakeSyncableService::SetAttachmentService(