[Sync] Support commit only types.

components/sync/user_events/user_event_sync_bridge.cc

Index: components/sync/user_events/user_event_sync_bridge.cc
diff --git a/components/sync/user_events/user_event_sync_bridge.cc b/components/sync/user_events/user_event_sync_bridge.cc
index 51e058ad5bd374c12cbd831b29963a3eb1d3b1bf..f601791b85c593b3c855c669e410b7b26746c723 100644
--- a/components/sync/user_events/user_event_sync_bridge.cc
+++ b/components/sync/user_events/user_event_sync_bridge.cc
@@ -84,7 +84,15 @@ base::Optional<ModelError> UserEventSyncBridge::MergeSyncData(
 base::Optional<ModelError> UserEventSyncBridge::ApplySyncChanges(
     std::unique_ptr<MetadataChangeList> metadata_change_list,
     EntityChangeList entity_changes) {
-  NOTREACHED();
+  std::unique_ptr<WriteBatch> batch = store_->CreateWriteBatch();
+  for (EntityChange& change : entity_changes) {
+    DCHECK_EQ(EntityChange::ACTION_DELETE, change.type());

[Patrick Noland, 2017/06/01 21:34:40]

Are you sure you just want to DCHECK this condition? Maybe it would be better to
just ignore non-deletes? Imagine you try to delete an entity when the change
type was ADD and fail to do so, are you really in an error state?

[skym, 2017/06/01 22:43:20]

The processor should never send us any other action. We don't have any handling
for any other action. You're right that we could ignore it.

Even if we change our minds about USER_EVENTS in the future and start sending
remote changes down to chrome clients, these old clients aren't going to see
that data. We don't currently have progress markers, we don't currently include
USER_EVENTS in GetUpdates calls. I think we're safe to make changes without
locking ourselves in here with the DCHECK.

It's difficult to reason about what the correct handling of an ADD should be,
because it shouldn't happen. Deleting events is low impact anyways.

+    batch->DeleteData(change.storage_key());
+  }
+  batch->TransferMetadataChanges(std::move(metadata_change_list));
+  store_->CommitWriteBatch(
+      std::move(batch),
+      base::Bind(&UserEventSyncBridge::OnCommit, base::AsWeakPtr(this)));
   return {};
 }