Sync currently has two different garbage collection behaviors on the client:

(1) aging out sync entities
This is currently done for Typed Urls , Device Infos, Autofill (autocomplete). These are only used to get the client's sync data base in-sync with the clients native feature DB. For example, the history backend deletes typed URLs after 180 days but the bridge doesn't tell the sync processor about those deleted. That's why the server tells the sync to gc metadata for typed URLs with an age > 180d.

(2) Support full read semantics
For Wallet, we use GarbageCollectionDirective.Type.VERSION_WATERMARK to implement non-incremental sync protocol (ie., full replace)

http://crrev.com/c/1183902 changes the client behavior for GarbageCollectionDirective.Type.VERSION_WATERMARK directives to always clear the data + metadata. However, it seems somewhat hacky to have this behavior for one specific GC directive, and not for others.

That CL also ignores the specific watermark passed in that directive, and always deletes the entire client storage.

We should reconsider the way GC is done in Sync to see if we need these both ways, and if there's a more elegant way to cover the use cases.

If we do decide to keep the current approach, we should at least rename the watermark field for VERSION_WATERMARK directives to make clear that the value isn't read.